"Bayern Munich are very strong," The United forward told the club's official website.
"I watched them play against my old team, Sporting Lisbon, and I was very sad because I didn't expect Sporting to lose so heavily [12-1 on aggregate].
"But this can happen in football. Bayern are very good - they have mature players, strong players, quick players. Franck Ribery and Miroslav Klose are very impressive.
"Bayern play like a team and they look very difficult to beat."