Turnbull played only a handful of games, mainly watching from the bench as understudy to Petr Cech, before joining Doncaster in the summer.
He said: "I had four fantastic years at Chelsea but I just want to play. It was time to move on.
"I am happy to be here. It was amazing being at Chelsea but it's not the same as playing.
"And I already feel the benefit from starting week-in, week-out."
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