The Belgian side are hoping to go through after securing a 1-1 draw at home in the first leg.
But they must go to Old Trafford without Dennis, who was the man of the match in the first leg.
Another who is out of the game is defender Eder Balanta, who is suspended.
Krepin Diatta is another doubt, as the Senegalese winger may not recover in time to take his place in the 18 man squad.
But Dennis is the biggest absence, as he has nine goals in 31 games for Brugge this season.