There is no doubt that this match is the biggest of David Moyes' career.
Despite the continued hurt in the league and the embarrassing cup exits, and the post-match media grillings, this match in Greece represents the manager's final chance to taste success in 2014.
Moyes has suffered this season, and much of it has not been his fault. However, the one thing he is responsible for above all else is tactics. Even the fitness of the players is delegated to medical staff who more capable in this field than your average UEFA qualified football coach.
Moyes chooses the formation, and he must live and die by his choices.
Recently, we have seen United return to a 4-4-2 at times, as the manager tries to find a way to accommodate his new stellar signing, Juan Mata.
In just a handful of matches, we have seen the Spaniard played wide out on right-wing, to his more comfortable and preferred free role behind the striker. The Crystal Palace victory was hopefully a sign of things to come, as United set up 4-2-3-1 and tried to pass the ball.
United achieved an impressive 90 per cent pass completion and only tried to cross the ball 19 times, per Squawka. Compare this to the 81 crosses they put in against Fulham recently, and it points toward maybe a light-bulb moment occurring in Moyes' head.
United were much more economical against Palace than versus Fulham, and I hope to see more of the same tactics against Olympiakos.
Utilising Marouane Fellaini
The truth is many Red Devils fans do not rate Marouane Fellaini, and this is hugely unfair to the player.
People will claim that he "is not United class" despite him coming 12th in Bloomsberg's statistical analytics poll of the best players in Europe for 2013, as previously reported by Ben Curtis of The Mirror.
Fellaini is not a bad footballer, and David Moyes knows this better than any supporter, but he does need to find his place at Man United and earn his spot.
The Belgian's return to United's starting line-up on Saturday was a successful venture before his inevitable substitution. His heat-map shows excellent fluidity across the midfield and points toward his future use as a deep-lying player to partner Michael Carrick.
If Moyes persists, as he should, with 4-2-3-1, then Fellaini has a definite place in his midfield. The lanky Belgian does not have the mobility to play in a 4-4-2 and does not have the flair to skip past players in the centre.
As we saw against Palace, Fellaini was able to play off a deep position but with freedom. He was not playing a truly classical defence role, but he held his position when he had to and linked the play with Rooney and Mata when he had the opportunity.
He also had arguably the best couple of chances to score of the game, and as his fitness improves, you can expect to see his name among the goalscorers. Imagine that: A Man United midfielder that actually scores goals.
Fellaini will be of great help to Moyes during this transitional period and against Olympiakos. The player knows he has to play, and play well, before this summer's World Cup, so the incentives are all there for him.
He could thrive where Anderson and Tom Cleverley have failed. With backup from Darren Fletcher, the temptation to overplay Fellaini should not be an issue.
Januzaj or Young?
As a question, there is little doubt whom United fans would pick to be Antonio Valencia's wing partner on Tuesday.
The impact of Adnan Januzaj has been felt across the world, as United display their new boy wonder. He has been impressive since his first match and can simply do things that other players cannot.
However, at 18 he is still a boy and not yet wholly reliable in a defensive state. There are certainly signs of improvement in his positioning without the ball, but on a vital night for Moyes, he may just go with the older and more experienced option.
Much like Fellaini, Ashley Young is maligned by fans from coast to coast. However, his form since his return from injury has been very good, if not mindblowing.
He has contributed well from the left-wing and has a good understanding with Patrice Evra.
Moyes may feel that Young will offer him balance while still being able to join in with the attack.
With Mata missing, it all depends on how the manager plans to break down the Greek team on the night.
If Young is included with Valencia, we are more likely to see a 4-4-1-1 being used, as Moyes clings on to the formation he keeps reverting to.
However, if he does plump for 4-2-3-1, the extra innovation provided by Januzaj could be what makes his mind up and forces his hand.
It all depends on how brave the manager is come kick-off on Tuesday.
Unleashing Van Persie
It is impossible to measure how much Man United have missed their alpha male in attack.
As Rooney continues to take the plaudits, Robin van Persie is the difference in the opposition's final third.
United have bled pint after pint of blood in his absence, and despite the form of Danny Welbeck through the season, he simply is no RvP.
Van Persie is the bullet in David Moyes's gun, and no bullet equals no kill. Moyes' task now is to get van Persie fit and firing.
It is a myth to consider the player's age, for he is not old. If United can find a style of play where they get the ball to van Persie's feet as often as possible in the penalty area, they will score goals and they will win matches.
What formation will David Moyes play against Olympiakos?
Rooney is destined to play in the hole behind van Persie against Olympiakos, and the chemistry between the two will be crucial to the result.
United have looked so much more comfortable away from Old Trafford this season, so we can expect some good interplay between the two players and the adjoining Fellaini.
When United get it right they explode, just as they did when they visited Bayer Leverkusen.
The Dutchman is the heartbeat of United's goals-scored tally, and they will be hoping he moves swiftly though the gears on Tuesday evening.