Bopp who made 29 appearances for the Millers before picking up an injury which kept him out of the squad since the end of Febuary.
The Ukraine born midfielder joined the Millers on a one year contract after impressing on trial last pre-season, but today has signed for Crewe on a 3 year contract.
Outgoing Crewe boss Diaro Gradi was pleased to get his man, speaking to the BBC he said; "He's what I call a proper footballer. He was once a high-flier at Bayern Munich, his career has floundered a bit but he had a good year at Rotherham."
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