Kylian Mbappé vs Victor Lindelof and Chris Smalling
If this tie is to be decided by any one player, it will most likely be the one who can exploit any half-yard conceded or slight lapse in concentration.
Kylian Mbapppe did exactly that in the first leg, ghosting in behind Eric Bailly score Paris Saint-Germain's second away goal and give Manchester United much work to do in the French capital.
Bailly is likely to be left among the substitutes for this second leg, with Chris Smalling back to full fitness and impressing alongside Victor Lindelof in the centre of United's defence.
If, between them, they can take responsibility for tracking and shackling Mbappé, then United have will have a far better chance of progression. United need to score at least twice in Paris, though keeping a clean sheet is just as important.
But the dynamism and movement of Mbappé, which caused United so many problems in the first leg, is likely to trouble them once again.
Marquinhos vs Scott McTominay
If needing to turn around a 2-0 deficit in Paris was not difficult enough, United must do so with a depleted midfield. Nemanja Matic and Ander Herrera are injured, while Paul Pogba is suspended after his sending off late on in the first leg.
Between them, Pogba, Matic and Herrera could not prevent Marquinhos from running the show three weeks ago. The natural-born centre-half has successfully stepped up into midfield under Thomas Tuchel and he dictated the play at Old Trafford, particularly as PSG took hold in the second half.
Just over a year from his surprise Champions League knock-out stage start in Seville, it will be up to Scott McTominay to win the battle for supremacy against Marquinhos in the middle of the park.
McTominay signed a new contract recently but still has something to prove under Solskjaer, having been promoted to the first team and championed by Jose Mourinho. His performance against Liverpool showed he can help to shut down a elite side.
The Parc des Princes in the second leg of a Champions League knock-out tie will be a different proposition, though. Much of United's success will be depend on how he, Fred and Andreas Pereira perform.
Angel di Maria vs Ashley Young
The duel between Angel di Maria and Ashley Young at Old Trafford provided much of the evening's intrigue, and it appeared to be one that Young was winning when he sent his former United team-mate crashing into the advertising hoardings with one over-zealous shoulder barge.
But it was Di Maria who would have the final and decisive say, setting up both Presnel Kimpembe and Mbappé's goals in the second half, then screaming expletives into the Scoreboard End.
If Tuchel was looking for United players to target and exploit before the first leg, a 33-year-old right-back who spent the majority of his career as a winger would have stood out.
Young has adapted well to his new role, particularly when it was thrust upon him at the start of last season, but he is liable to caught out at this rarefied level against an in-form, elite opponent.
Last time, Di Maria showed Old Trafford what he could have brought to United had his year-long spell worked out differently. A repeat performance in Paris may end United's hopes of progression once and for all.
