With two matches to play former champions Manchester United and Porto and impressive newcomers Malaga are the only teams mathematically certain of qualifying, although the make up of the first knockout round will become clearer this week.
UEFA use a complicated head-to-head and mini-league tiebreak system to work out the final rankings instead of goal difference, meaning uncertainties about who qualifies can last until the final kick of the final match.
But in reality, Arsenal, Barcelona, Bayern Munich, Celtic, Borussia Dortmund, AC Milan, Paris Saint-Germain, Real Madrid, Schalke 04, Shakhtar Donetsk and Valencia can all advance this week if results go their way.
Chelsea will also go through on Tuesday if they win their Group E match against Juventus in Turin, sealing their place in the last 16 for a 10th successive season.
But Roberto Di Matteo's men travel without a win in their last four Premier League games and reeling from a surprise 2-1 defeat at West Bromwich Albion on Saturday when defensive lapses cost them the points.
"For some unknown reason November seems to be a bad month for this club," the Italian told reporters.
No European champion has failed to get through a group stage since the Champions League began but if Chelsea lose in Turin they will be in danger of becoming the first to do so.
Chelsea's most important victory in recent weeks came two weeks ago when they beat Shakhtar 3-2 at Stamford Bridge with a last-gasp header from Victor Moses sealing the points and making qualification more likely.
Their domestic form has been poor with only two points from 12 in their last four games, but at least their Champions League prospects are far brighter than those of English champions Manchester City.
City rose to the top of the Premier League for the first time this season with their 5-0 demolition of Aston Villa on Saturday yet their Champions League form has been dreadful, losing at Real Madrid and Ajax Amsterdam and only drawing at home with Dortmund and Ajax last time out.
They are bottom of Group D with just two points, behind Borussia (8), Real (7) and Ajax (4).
Captain Vincent Kompany is at a loss to explain their failings in Europe.
Speaking on BBC TV's Match of the Day this weekend, he told presenter Gary Lineker: "If we had the answer to that we probably would not be where we are.
"But for some freakish reason, we still have a chance in the Champions League and I would like to think that in the two games that are left we can pull off a big surprise and qualify as we still have a chance."
Real have a long-enduring European rivalry with Manchester United going back to the 1957 European Cup semi-final, but had never met City before until this season and will head to the Etihad on Wednesday as favourites.
However, Real's French striker Karim Benzema believes his compatriot Samir Nasri is a big threat to the Spanish champions' chances of sealing the victory that could see them through.
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