The Swede has not featured since suffering a serious knee injury in United's Europa League quarter-final clash with Anderlecht in April.
Ibrahimovic is not expected to be available for selection until the turn of the year and he will then need to try and reclaim his starting spot from summer signing Lukaku.
United have already fallen well behind rivals Manchester City in the title race, trailing by eight points after just 11 games, but Lukaku feels Ibrahimovic could help them to bridge that gap.
Lukaku told Sky Sports: "It motivates me really, because he's another good player to add to the squad.
"When you face a team like us it's difficult for opponents. Ibrahimovic coming back will help us chase Manchester City."