Mumbru, 46, who was admitted to hospital with acute pancreatitis the day before the start of Eurobasket, has still not fully recovered, leading to his decision not to coach any more of Germany's games at the tournament.
He was back on the bench in the last 16 against Portugal, but visibly unwell and with a noticeable weight loss.
"In the last few days, I have realised that I am not physically ready to lead the team from the sidelines during games," admitted Mumbru, who, although he will be sitting on the sidelines accompanying the German team, will leave the main functions in the hands of his assistant for the remainder of the tournament.
"I have decided to adjust the functions of the coaching staff and give Alan the responsibility of head coach during the games," explained the Spaniard. " I think this is the best way to help the team achieve success," he added.
Germany's next game is on Wednesday in the quarter-finals against Luka Doncic's Slovenia.