The Argentine superstar's fitness is crucial if Barcelona are to have any chance of staging a recovery from their crushing defeat in the first leg. Messi looked back to his old self against Athletic Bilbao on Saturday as he came off the bench to score one and assisted another and he will be desperate to make up for his lacklustre performace in Germany.
The former Real Madrid winger was outstanding in the first leg as he tormented Jordi Alba all night and was involved in three of Bayern's four goals. With Barcelona going all out for a big victory in the return leg, Robben's electric pace will be an even greater threat to the Catalans on the counterattack.
Barca came back from the dead against AC Milan in the round of 16, but it will take something truly spectacular to repeat the feat against this Bayern side. Indeed, such is the amount of confidence the visitors will have drawn from their total superiority in the first leg that they will step out at Camp Nou expecting another victory.