The midfielder signed for the Toffees on deadline day last Monday.
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Schuster told Kicker: “I didn't foresee anything… (the move) came about at very short notice.
"On the one hand, we would have liked to keep Merlin..: (but) I think it was clearly stated that an offer of a certain size could be an argument for letting the boys go."
Rohl joined Everton for an initial €2m loan fee for the season, with a permanent option set at €12m.