Schurrle has struggled for form since his January return to Germany.
"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."
Schurrle, who has been dropped to the bench, added: "I have not done much in the past few weeks and I realise now that I am stuck in a hole."