"I believe Wigan will sell Antonio for the right offer and they will decide that once they receive that," agent Diego Herrera told skysports.com.
"Real Madrid made an offer last week, but nobody from Wigan has told me of an offer from Manchester United.
"There are a number of interested clubs and I will speak with Wigan about it this week.
"Once the season is over I expect offers to be made and I will sit down with Antonio and decide what offer is in his best interests."
Wigan boss Steve Bruce admitted recently that holding to Valencia this summer would be impossible, amid the wide interest in his services.
The 23-year-old has been one of the best performers for the Latics this season and the interest from Madrid has been growing since the winter break.
Wigan have already sold Wilson Palacios to Tottenham and Emile Heskey to Aston Villa, while Amr Zaki's loan at JJB is set to end.