(CNN) -- Manchester United manager Alex Ferguson says David Beckham will offer a "threat" to his side's Champions League ambitions but admits he has no idea if his former midfield star will feature in the AC Milan lineup at Old Trafford.
Beckham's return, seven years after leaving United, has been greeted with considerable fanfare and he was mobbed at Manchester airport, but Ferguson believes his participation in the last 16, second leg tie is in doubt.
"It is difficult to say what effect David will have on the tie because we don't know for certain whether he is going to start," he told gathered reporters.
"He has not started a game since the first leg against us. I find it difficult to pick their team.
"It all depends on Pato's fitness, but he has traveled. I don't know if Beckham will start. I haven't got him down in my conclusions," he added.
Should he turn out, with Milan trailing 3-2 from the first leg in the San Siro, Ferguson will be wary of a storybook return for the veteran England international.
"I think David's strength has always been in his crossing and his set-piece play. That doesn't change. That has been his strength throughout his career, and the one thing with David that you have to be aware of is his delivery of the ball.
"It is fantastic and that is a threat. There are lots of threats that Milan have, of course, but we are well aware of that posed by David. If he plays, that will be the threat against us."
Milan coach Leonardo would not give a guarantee that Beckham will start at Old Trafford.
The 34-year-old has not started a game since the first leg three weeks ago when he was substituted in the second half as United took control control of the tie.
"For Beckham, it is obviously something special to have the opportunity to play against Manchester United given his history at the club," he told reporters.
"When we saw the draw we knew it would be an unbelievable situation for him.
"But knowing David, and his attitude, I am sure he can cope. And independent of that, he is very important for the group as a whole and we are always grateful for his presence."
Leonardo is waiting on the fitness of Brazilian ace Pato, who has shown fine recent form in their mounting challenge in Serie A to defending champions and arch-rivals Inter Milan, before deciding on his team.
United will be grateful Rooney has been passed fit to start after a minor knee injury.
His two goals in the first leg turned the tie around after United fell behind early and Milan skipper Massimo Ambrosini knows only too well the threat he poses.
"He is better than in the past. He has improved and is now a complete player. He is one of the best in the world at the moment."