The 24-year old winger returned to the Bundesliga in January but has appeared just four times for Die Wolfe because of injuries he attributes to the training methods of the Blues.
"At top English clubs during the season, the players are hardly loaded with any training, since it's almost all about regeneration and tactics." he said.
"In England you can get your fitness through the season with the games.
"I'm still missing the last per cent. In January, I almost had no training between the change of clubs because of back problems."
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