Pato sustained a hamstring injury last week and was forced to miss Brazil's friendly win over Ireland on Tuesday as he looks set to miss the crunch Serie A clash.

"Pato is a great champion," Milan vice-chairman Adriano Galliani was quoted by Italian media.

"His absence will cause us some problems but we hope he will recover in time for the game at Manchester United."

Milan face an uphill battle when they visit United in Tuesday's last 16 return leg, following a 3-2 first-leg defeat at San Siro.

Thrilling

Despite the setback of Pato, coupled by the injury of Roma star Francesco Totti, Galliani anticipates a mouth-watering encounter between the close rivals.

"Despite the absence of Alexandre Pato and (injured Roma captain) Francesco Totti, from an aesthetic level, this game is the best that our tournament can offer," added Galliani.

"Roma and Milan play good football and I expect a thrilling game with the stadium set to be sold out."