The likes of Anthony Martial and Marcus Rashford have failed to impress in Lukaku's absence, with Mourinho slamming the duo for underperforming.
Both will start at home to Watford tomorrow though as Mourinho wants all his squad to have playing time ahead of the final. David De Gea will be rested, so it gives a chance for Sergio Romero to play.
"Sunday is again for Martial and Rashford. The only problem was I also knew he didn't want to play me", Lukaku said during an interview with DHNet. "I have every player apart from [Marouane] Fellaini and [Romelu] Lukaku available".
However, it seems now that although it's not a long term injury, it could still mean that Lukaku will be forced to watch from the bench.
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United, despite finishing in their highest-ever position since Alex Ferguson retired, have not impressed fans with their displays so far and the FA Cup is their last chance of lifting any silverware this season.
Michael Carrick will start as captain against Watford in his final appearance for United before joining Mourinho's backroom team for next season.
The Belgian striker has been out of action since he injured his ankle in a clash with Arsenal centre-back Konstantinos Mavropanos during United's 2-1 victory over the Gunners a couple of weeks ago. "We are waiting to see if he can play in the final - if not starting at least to be on the bench".
Then comes the little matter of World Cup preparation for the forward, who is definitely enjoying life more under current Belgium boss Roberto Martinez and his assistant Thierry Henry.