London : After one of the most unpredictable Champions League matchdays in years, some of Europe's big names have reputations to restore and ground to make up this week as the group stage approaches crunch time.
While Manchester City already is in must-win territory, AC Milan, Juventus, Bayern Munich and Chelsea find themselves in precarious positions heading into the fourth round of matches.
Of that stellar quartet, Chelsea is in most danger and a failure to beat Shakhtar Donetsk on Wednesday could leave the English giants on the brink of becoming the first defending champions to miss out on qualifying from the group stage.
"It's fair to say that," Chelsea manager Roberto di Matteo said, when asked if this was the team's biggest match since last season's Champions League final. "We'll have to produce a special night on Wednesday, for sure."
Shakhtar, driven on by its talented Brazilian contingent, has won all 14 of its matches in the Ukrainian league and is top of Group E by three points after the 2-0 home win over Chelsea last time out. That came in a surprise-filled round that saw Milan, Man City, Arsenal and Real Madrid also lose, Juventus held to a draw at Danish minnow FC Nordsjaelland and Barcelona and Manchester United requiring late goals to seal comeback wins at home.
Chelsea is second in its group - a point ahead of Juventus, which will look to rebound from seeing a 49-match unbeaten domestic run end this weekend when it hosts Nordsjaelland.
"The world isn't over and Juventus isn't finished," goalkeeper Gianluigi Buffon said after the side's 3-1 loss to Inter Milan on Saturday. "We've got a lot of positive things to look forward to. Wednesday is a very important match and we can't let ourselves be distracted by this result."
Italian rival Milan - a seven-time European champion - is second in Group C on four points and seemingly chasing the runner-up spot behind Malaga ahead of Tuesday's match against the Spanish side, which has won all three matches. Zenit St. Petersburg is a point behind Milan and plays Anderlecht away next.
Bayern, meanwhile, is engaged in a three-way fight with Valencia and unheralded Belarusian side BATE Borisov for two qualification spots in Group F. All three teams have six points, with Bayern - last season's beaten finalist - currently third but facing a home match against last-place Lille on Wednesday.
Even Madrid is far from sure of advancing after losing 2-1 at Borussia Dortmund two weeks ago to slip behind the Germans in Group D but things look much rosier for fellow tournament giants Barcelona and Manchester United, who are protecting perfect starts and are both seeking to go through with two games to spare this week.
Barca, fresh off its best-ever start to a Spanish league campaign in its 113-year history, visits Celtic in Group G and United travels to Braga buoyed by going top of the English Premier League on Saturday.
As well as Malaga, FC Porto will also go through this week if it can secure a fourth straight win in Group A.
For Man City, the challenge is simple. The English champions, who have one point so far, need to win all their remaining matches - including a trip to Dortmund and the visit of Madrid - to avoid group-stage elimination for the second straight season. Ajax, which beat City 3-1 a fortnight ago, is first up for Roberto Mancini's side.
"It will be a miracle," Mancini said of his side's hopes of progressing.
Madrid can reclaim control of the group by beating Dortmund at the Santiago Bernabeu, where the Spanish champions have won seven Champions League games in a row - scoring 27 goals.
Elsewhere, Schalke hosts Arsenal in a meeting between the top two in Group B. Schalke won 2-0 at Emirates Stadium in the first match of the double header and Arsenal could be caught by third-place Olympiakos with a loss in Germany on Tuesday.