The two teams are on track to reach the final next week.
Berbatov wrote for Betfair: "Lukaku is a concern for me, he's scored 23 goals for Inter this season which is a great achievement.
"I always said I liked him as a player, that if he got chances he would score goals.
"He's very quick, and very strong, and his goal on Monday was spectacular. He's the main danger for whoever they play against.
"Inter also have Alexis Sanchez and Ashley Young, who will know United very well."If they play United in the final, these players will want revenge, to show United that they were wrong to let them go.
"Lukaku would love the chance to bang in some goals against the team that let him go. Lukaku took a lot of criticism in Manchester, and he was right to leave.
"Look at him now, he has been fantastic for Inter."
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