Clarke believes the central midfielder has not been quite able to match the exceptional form he showed during the first half of the campaign since an enforced mid-season spell on the sidelines due to hamstring and shoulder injuries.
"Certainly in the first half of the season his performances were top," said Clarke to the WBA website.
"Since he picked up a couple of injuries he has been a little bit less effective in the second half of the season, it's fair to say.
"It's been a long, tough season in his first season in the Premier League.
"It's a difficult season and the way he started was incredible. That showed the quality of the player.
"But since he picked up a couple of injuries and knocks, a little bit like the team in the second half of the season, I just don't think he's ever quite found his rhythm."