The hosts were thrashed 6-1 in their Champions League clash on Wednesday and their coach believes it was an important learning curve for his youngsters
Two goals each for Cristiano Ronaldo, Karim Benzema and Gareth Bale embarrassed the Bundesliga side in their Champions League last-16 first leg, with Klaas-Jan Huntelaar's fine late volley serving as scant consolation to the beleaguered hosts.
With Sead Kolasinac, Kevin-Prince Boateng and Roman Neustadter all suffering injuries during the contest, Keller was left with little to shout about at the final whistle - although he did stress the defeat would serve as an important learning curve for youngsters Julian Draxler and Max Meyer.
"We had a very young team on the pitch and they learned a lot," he told Sky. "Guys like Julian Draxler, Max Meyer, Sead Kolasinac or Joel Matip know now that at this level you have to be even more focused and you are allowed even fewer mistakes.
"Kolasinac is at the hospital, maybe he has a broken jaw. Kevin-Prince Boateng is injured, Roman Neustadter has knee problems. It was a s**t game plus we have all these new injuries now.
"We started well until we conceded the first goal. Then we made error after error and a team like Real Madrid exploit that without any mercy."
The second leg takes place at Santiago Bernabeu on March 18.