The Sweden hitman has been limited to two starts and 14 substitute appearances in the Barclays Premier League since his summer arrival from Werder Bremen.
"Markus is very disappointed things haven't worked out for him so far," Clarke told the WBA website. "He hasn't had enough minutes on the pitch.
"Some of that can be my fault and he can be unhappy with me. He shouldn't be unhappy with himself.
"He trains every day, he's a consummate professional and he is well liked in the group.
"Having that type of player in your group is important because not every player can play every minute, not everyone can be a regular in the squad."