The Spanish midfielder has signed a four-year deal having allowed his contract to run down at Old Trafford.
And in other news, Manchester United are in talks with Andreas Pereira over a new contract.
The midfielder, 23, saw plenty of first-team action under Ole Gunnar Solskjaer last season and is set for a reward.
The Daily Mail claim discussions are already underway with the Brazilian, who has one year left on his current deal.
The report adds that David de Gea, Axel Tuanzebe and goalkeeper Dean Henderson have also been offered fresh contracts.