Jose Mourinho's men are currently second in the Premier League, eight points behind their bitter rivals.
They will be boosted by Ibrahimovic's return in the coming weeks. The Swede — who scored 28 goals in 46 games in his first spell at Old Trafford last season — signed on for another year earlier this season as he makes his way back from knee surgery.
"It motivates me really, because he's another good player to add to the squad," Lukaku told Sky Sports .
"When you face a team like us it's difficult for opponents. Ibrahimovic coming back will help us chase Manchester City."
Lukaku has scored seven goals thus far this season, and also provided three assists.