After five games and five victories, Manchester United are through to the knockout stages of the Champions League.

Only two other teams—Chelsea and Barcelona—went unbeaten in Group play. Although United's Group F opponents weren't exactly contenders, the Red Devils' wins in Kiev and Lisbon were plenty impressive—and Alex Ferguson's boys look to be a sure bet for a strong showing in Europe.

That said, two key factors might hold them back.

The first is the play of Cristiano Ronaldo. Ronaldo has netted 14 goals in 18 games this season, including five in his club's five European games. He's peaking, in other words—but is he peaking too early?

If Ronaldo flames out prematurely, United will need big help from Carlos Tevez, Wayne Rooney, and Louis Saha. Tevez and Rooney have been serviceable, and the ever-unfit Saha has managed to contribute—but at least one of the three will have to elevate his game when (or if) Ronaldo wanes.

The second question mark, meanwhile, is United's shaky defense. The Devils have conceded just 13 goals this season, but five of those have come in their last seven games.

Wes Brown has been the club's most consistent defender, but he's mired in a distracting contract dispute. Rio Ferdinand and Nemanja Vidic both started strong but have since been prone to lapses in concentration.

Against the likes of Kaka, Ronaldinho, and Zlatan Ibrahimovic, even the slightest letdown can be disastrous.

My own opinion is that United's Champions League fate will depend entirely on their domestic performance. The better they do in the EPL, the less likely they'll be to excel in Europe.

The Devils arguably have enough depth to compete on two fronts, but their new players lack big-game experience. Then again, if the Portuguese Prodigy plays up to his full potential, Sir Alex might just land the second European title he needs to cement his place in United folklore.