The Spaniard sealed a club record £37.1m move to Old Trafford from Chelsea on Saturday evening.
"I don't think we'll have any more in January," he told the club's official website.
"I'm still working on it and looking to do things – we know exactly what we'd like to do and if it changes in the last week, which sometimes it can do, then we'll try.
"I hope it will be the first of some new signings that will arrive, but more likely in the summer."
The 25-year-old could make his United debut in Tuesday's visit of Cardiff City.