The German said: "John and Frank have been injured for a long time and it was a big game against Olympiacos so it was a decision from the coach.
"I'm not sure if anyone is untouchable now. I think they are both still very, very important players for us but you have to be careful after injuries.
"I know this myself, having had eight months out. It's not an easy time when you come back - you can't play every game straight away.
"The coach has a lot of experience. He knows what he's doing."