Vincent Kompany was rushed to hospital on Friday afternoon. A medical helicopter also landed at Bayern Munich's training ground for the Belgian manager, BILD reports.
Already unwell this week, Kompany had been struck by a viral infection last weekend. It forced him to lie down in his coach's room during and after the match against RB Leipzig (6-1).
He then missed Bayern's training sessions on Monday and Tuesday, leaving his assistant to handle the press conference ahead of the Champions League match against Arsenal (2-2).
According to BILD, the viral infection has caused problems with "old scar tissue". The former Manchester City player is struggling to move around at present, among other issues.
The Belgian said at the press conference after the match against Arsenal: "As a coach, you always want to do everything and achieve everything. I caught a pretty nasty virus last week and, of course, I didn't listen to my body. At a certain point, my body said: Stop now!"
Why an ambulance took Kompany to hospital on Friday remains unclear. According to BILD, there is "great concern" for the Bayern manager.