Forwards
Alexis Sanchez

The Chilean was left out of the starting line up against Young Boys. He did not feature in the match entirely. He will be fresh for this tie if given the nod. In 22 matches, he has scored only three goals for the side. Despite his poor return, he still has a lot to offer the team.
Sanchez is favoured to take up the left wing position. He had a few chances against Watford and there is genuine belief that he can deliver the goals with time. With Marcus Rashford suspended, he has the chance to prove doubters wrong.
Anthony Martial/Jesse Lingard

At the moment, both seem to be performing at a similar level. Lingard has not fully hit his stride since England's solid World Cup run. Lingard has the advantage in this situation given his ability to distribute the balls while on the move.
Martial has raw pace and great dribbling ability on the other hand. He greatly improved in the second half of their Champions League tie and even got a goal. Mourinho may have seen enough to give him another bite at the cherry. On a good day, Martial can be ruthless even in the face of transfer speculation.
Romelu Lukaku

He is undoubtedly United's main man up front. He has played as the sole striker on many occasions with supporting wingers. Lukaku has started the season strongly in terms of his goal return. He was the top scorer last season with 27 goals and the current style of play dictates that he will have more chances come his way.
On long balls, Lukaku can bully many defenders and win aerial duels. He is also one of the target men during set pieces. Regardless of the formation, his primary objective will be to score or at the least create chances for his teammates.