The Belgium international is now at Inter Milan, having moved clubs prior to the closing of the Premier League transfer window.
"Lukaku hasn't been able to fulfil his potential at United and I think it's down to his self-motivation and confidence," Saha said to Bwin.
"He should be a defender's nightmare, he's blessed with both strength and pace which is rare.
"He should be able to hold the ball and bring other players into the game, but he doesn't use his strength to his advantage.
"He has been a little unlucky, as he's not the type of player that can create a goal by himself, he needs service.
"United have struggled to get the best service into the area since he's been there and it means he can't do what he's best at – getting on the end of crosses and through balls."