As far as some clubs are concerned, their 2015-16 Champions League fate has already been decided.

There are those that have already sealed their place in the knockout stages, while others know that their interest in the competition will not extend beyond Christmas.

Nevertheless, there is plenty left to be sorted out, with some of the tournament's biggest names still not guaranteed a place in the last 16.

Here, Sports Mole has selected five things to keep an eye out for over the course of Tuesday and Wednesday night.

D-Day for English trio

Those of the belief that the Premier League is the best in the world may be forced to reconsider that viewpoint come 10pm on Wednesday evening. In fairness to Manchester City, they have done their bit when it comes to flying the flag, having already advanced through what was arguably the toughest of the four groups that the English clubs were dealt.

As for the other three, it has not been quite so plain sailing. Manchester United are first up on Tuesday and Louis van Gaal's men know that a win away at Wolfsburg will secure them a place in the next round. Unfortunately for the Red Devils, their German opponents are a formidable force at the Volkswagen Arena, where they have lost just once in their last 30 outings. Were United to slip up, that opens the door for PSV Eindhoven.

Chelsea are in the box seat in Group G, with a win against Porto enough for them to go through. A defeat, coupled with a Dynamo Kiev win over Maccabi Tel Aviv, would knock out Jose Mourinho's men. Meanwhile, Arsenal need to win by two clear goals or by a 3-2 margin away in Olympiacos to progress.

Neville's big night

News of Gary Neville's appointment as the new head coach of Valencia came from so far out of left field that it will probably not completely sink in until he appears in the Mestalla dugout on Wednesday night. Knowing the type of character that Neville is, he probably would not have wanted it any other way, but he has certainly been thrown in at the deep end.

Having taken up a watching brief for Saturday's 1-1 draw with European champions Barcelona, the 40-year-old's first game in charge is a must-win encounter at home to bottom-place and eliminated Lyon. Victory may not even be enough, given that if Gent overcome Zenit in the group's other match, it will be the Belgians heading through.

Even so, the magnitude of the match has not been lost on Valencia's young midfielder Danilo Barbosa, who said: "I think the team will make the most of that game. We all know Wednesday's game is one that will define our lives. The first contact with the new coach was very brief but he was a great player for Manchester United. Now it is time to pull together."

Joy of six for Zenit

In their six Champions League campaigns to date, Zenit St Petersburg have not exactly set the world alight. The Russian outfit have reached the round of 16 twice, but that is as good as it has got.

But, under the guidance of former Chelsea and Tottenham Hotspur boss Andre Villas-Boas, perhaps the Sky Blues are ready to make a significant impact at European football's top table. Fives wins from five outings has seen Villas-Boas's men qualify as group winners, while 12 goals for and four against during that time is an impressive return.

Now, they will be looking to join only a select few teams that have triumphed in all six of their group encounters. AC Milan (1992-93), PSG (1994-95), Spartak Moscow (1995-96), Barcelona (2002-03), Real Madrid (2011-12), Real Madrid (2014-15) have all done it and if Zenit can get the better of Gent - and help Neville's cause in the process - they will also have achieved group stage perfection.

Group E's three-way fight

It is little surprise that holders Barcelona are over the hill and far away in Group E. Having collected 13 points from a possible 15, scoring 14 goals and shipping just three along the way, Luis Enrique's charges will go into the draw for the last 16 as group winners and the team that others will be desperate to avoid.

Yet, while the battle for top spot was virtually non-existent, the race for second place has been highly entertaining. Roma hold all of the cards and know that if they beat BATE Borisov in the Stadio Olimpico, it will be them that join Barca in the next round. However, should BATE claim maximum points, there is every chance that the Belarusians will be celebrating reaching the knockout stages for the first time in their history.

Sandwiched between those two clubs are Bayer Leverkusen, who take on Barca at the BayArena on Wednesday - a match that will be officiated by English referee Mark Clattenburg. If Roma fail to win against BATE and Leverkusen see off the challenge from their illustrious Spanish counterparts, they will progress.

Summit battle in Lisbon

Advancing through to the knockout stages as the second-placed team more often than not comes back to bite a team when the draw is made - just ask Arsenal, who in recent seasons have faced the likes of Bayern Munich and Barcelona.

That is why Benfica and Atletico Madrid will be going hell for leather to win their encounter at the Estadio da Luz on Tuesday. Both sides qualified on matchday five and find themselves locked on 10 points. The advantage is with hosting Benfica, who lead the head to head record thanks to their unexpected 2-1 win in the Spanish capital earlier in the campaign.

It means that if Atletico are to win the group, only a victory will do in Lisbon. Diego Simeone's men will certainly head into the fixture full of confidence, having not tasted defeat in any of their last 12 matches. They also go into the game on a six-match winning run, during which they have conceded only two goals.

The permutations for each of the eight groups can be found here. Meanwhile, the current standings are available here.