Bayern may have a 1-0 lead thanks to an early strike from David Alaba, but the injury to Kroos will be a terrible blow. Arjen Robben was the man to replace him, with Thomas Muller moving into the centre. Kroos had been marking Andrea Pirlo during the game, so it will also disrupt Bayern's gameplan.
Kroos has been a key member of Bayern's squad during this season's campaign, and it will be interesting to see whether Robben is his replacement for the rest of the time he spends out injured.
It's not known whether it's merely a strain, or as severe as a torn muscle - the latter would likely keep him out for the rest of the season, so Bayern will be sweating over how serious it is. His loss would present a severe blow to their chances of victory in the Champions League, and certainly with this tie against Juventus.