The defender has left Stoke for Wolfsburg.
“Philipp will be a big loss for us," Adam stressed. “He was a big part of what we did last year and it's important we recognise that.
“I think he wanted to go back home to play, but he was well liked in the dressing room and will be missed.
“He was a top centre half. I liked him and on a personal note I'm sad to see him go."