Leverkusen were down after just seven minutes when Romano Schmid tapped in from close range.

With Leverkusen failing to click, coach Xabi Alonso brought Wirtz from the bench at halftime, but the Germany midfielder was brutally fouled eight minutes afterwards and limped from the field and left the arena on crutches.

Any injury to Wirtz, the club's talisman and best player this season, would be a serious blow to Alonso's side, particularly ahead of Tuesday's Champions League match against Bayern, who hold a 3-0 lead from the first leg.

"We'll have to wait and see," Alonso said after the match, adding "we don't have any results yet."

Bremen's Mitchell Weiser, who fouled the Germany midfielder, apologised after the match, saying "I hope it's not that bad -- I'm lucky it wasn't a red card."