"We have put a clause in the contract, so maybe he will come back to us at some stage," Ferguson told Pravda TV.
"We wish Giuseppe well and we know he will do well at Villarreal.
"It was a difficult decision to let him leave. I had several meetings with him to try and convince him that he would get plenty of football with us this season and all he needed was a bit of patience.
"But, having been on loan last season, he wanted first-team football all the time."