UEFAa s premier club competition was back underway this week, as three challengers for the coveted Champions League crown a Real Madrid, Bayern Munich and Chelsea a began their knock-out stage ties with contrasting fortunes. Holders Real Madrid cruised to a comfortable 2-0 win in Germany against Roberto Di Matteoa s Schalke, whilst Chelsea, Porto and Bayern Munich could only manage away draws against their less-fancied opponents.

Carlo Ancelottia s side is all but through to the quarter-finals after their away win in which star man Cristiano Ronaldo resumed normal service with a goal and an assist to his name. Meanwhile, Jose Mourinho and Pep Guardiola face tricky second legs at home in three weeksa time. Despite the somewhat unspectacular nature of the opening Round of 16 games, there were plenty of talking points across the four games played so far.

Here are four things we learned from the opening knock-out fixtures of the 2014/2015 Champions League:

1) Chelseaa s defensive uncertainty against a biga teams could cost them in the latter stages

An intense tactical battle at the Parc des Princes ended in a 1-1 draw a a result that suits the Blues far more than it does Laurent Blanca s Paris Saint-Germain side. There are two ways of looking at this game from Chelseaa s point of view: a resilient display away from home against quality opposition; or a lucky escape thanks to Thibaut Courtoisa heroics. The truth, perhaps, lies somewhere in between. There is, however, a concerning pattern emerging from Chelseaa s performances against top quality opposition this campaign. The Blues have only managed draws against Manchester United and Manchester City (twice) in the league, Liverpool in the Capital One Cup semi-final first leg and now PSG in the first leg of the Round of 16 despite taking the lead in these games.

In Mourinhoa s first spell in charge at Stamford Bridge, teams knew that going a goal down to his Chelsea side was tantamount to losing the game a the sheer defensive excellence of that team, combined with its lethal counter-attacking ability meant that recovery was near impossible. Despite the upgrades Chelsea have made this season in an attacking sense, the defence is still a concern. Edinson Cavani was allowed a criminal amount of time and space to head the ball into Courtoisa net to level the scores, and more than once, Laurent Blanca s side was able to penetrate the Chelsea defence with apparent ease.

Mourinho explained after the match that five of his players were unable to train prior to Tuesday nighta s game, which might partially explain the lacklustre defensive performance by the Premier League leaders. However, Gary Cahilla s form has come under question this campaign, and he suffered another torrid night in Paris playing alongside captain John Terry. Something else that Chelsea might have to be careful about is the exploitation of their left flank, and Eden Hazarda s lack of defensive ability. PSG right-back Gregory van der Wiel found himself in very good positions throughout the game in the attacking third, and it was arguably the area which lost Chelsea the tie against Atletico Madrid in last seasona s Champions League semi-final. As good as Cezar Azpilicueta is in one-on-one situations, opponents doubling up on him with an overlapping fullback could lead to serious trouble down the line for the Blues a particularly since Eden Hazard, for all that he offers going forward, does tend to shut off defensively, allowing his man to make runs in behind with impunity.

2) Real Madrida s individual talent stands out when team isna t at its best

Real Madrid werena t at their best in their comfortable 2-0 win against Schalke on Wednesday night, and despite his goal and assist, neither was star man Cristiano Ronaldo. That perhaps is an indication of why Ancelottia s side are considered favourites once again in this seasona s Champions League. The Spanish giants are positively frightening when playing at their highest level a with the trio of Ronaldo, Bale and Benzema, Real Madrid are perhaps the best in the world at killing defences on the counter a but their individual quality means that they can also produce results in games where they do not perform to the best of their ability as a unit.

Reala s second goal on the night came courtesy of a right-footed strike to the top corner from the edge of the box from their left-back, Marcelo. This, just moments after Di Matteoa s side had struck the cross-bar and were looking threatening for the first time in the entire game. The sheer individual ability of the Real Madrid XI means that they have match-winners all over the park a more so than any other team left in the competition, bar perhaps Barcelona. Other top teams in the competition, including Bayern Munich, Atletico Madrid and Chelsea do have individual talent, but are arguably far more reliant on the teama s structure and performance as a whole.

The holders have one foot in the quarter-finals, and will no doubt improve in future games. However, the fact that they possess so much individual talent to bail them out when needed no doubt holds them in good stead for the latter stages of the competition whether they will face much sterner tests.

3) Bayern Munich increasingly dependent on Arjen Robben to provide spark in attack

It is not often that Bayern Munich fail to score in ninety minutes of football, and Tuesday nighta s 0-0 draw against Shakhtar Donetsk leaves Pep Guardiolaa s side with plenty to do in the return leg at the Allianz Arena. The Bavarian giantsa form has been somewhat indifferent after the winter break, with a heavy defeat against Wolfsburg preceding a draw at home to Schalke where, just like on Tuesday, Bayern were reduced to ten men. It was a frustrating night for the Bundesliga leaders, and it got no better when Xabi Alonso saw red in his 100th Champions League appearance for stopping a potentially dangerous counter-attack by hacking down Alex Teixiera.

There is no doubt that Guardiolaa s most salient test this season is the Champions League, with the 2012/2013 winners being bundled out in humiliating fashion by eventual winners Real Madrid last season. His main problem, yet again this season, might just be his sidea s lack of incisiveness and at times, the sterile nature of their possession. That is where Arjen Robben comes in. The Dutchman has arguably had the best 18 months of his career, and his form this season is a huge positive for Guardiola. The ex-Chelsea winger already has 16 goals to his name so far in all competitions, and the consistency of his performances means that he can genuinely lay claim to being the third best player in the world after Lionel Messi and Cristiano Ronaldo.

Robben has proved that he is capable of providing the attacking spark that his team is likely to need in the knock-out stages of the competition against difficult opposition. In sharp contrast to the possession-based, tiki-taka football Guardiola has become famous for, the Dutchman is capable of being very direct a running at defenders with the willingness to take them on, and either cut in from the right to shoot with his left foot, or beat his man on the outside for a pull-back or cross to a team-mate from the by-line. Despite the draw in the first leg, Bayern are huge favourites to go through a and it might just be Arjen Robbena s brilliance that takes them a step closer to winning the Champions League again.

4) Basela s dream run is likely to come to an end against Porto

The Swiss side defied the odds by finishing ahead of Liverpool and qualifying for the knock-out stages along with Real Madrid from Group B. To make things better for Basel, they got an extremely favourable draw for the Champions League Round of 16 a avoiding all the biggest names in the competition. However, Portoa s impressive performance and away goal in the first leg means that the Portuguese giants are now huge favourites to progress to the quarter-finals.

In a game Porto dominated from start to finish, Basel did manage to find the breakthrough, only to be pegged back by right-back Daniloa s second-half penalty. Perhaps the most talked-about moment of the game came when English referee Mark Clattenburg disallowed Casemiroa s strike, ruling that two Porto players had obstructed Basela s keeper and were thus in an offside position. Despite their heroics so far, the Swiss side face an uphill task when they travel to Portugal in three weeksa time, and they will need something of a miracle to progress to the quarter-finals.