Manager David Moyes, who signed Mata from Chelsea in January, is reportedly expected to have around £200m to spend on transfers.
The Spaniard has suggested that part of the reason why he moved to Old Trafford was because the club made promises that there would be new arrivals in the summer.
"When I joined, in all my conversations with the board and the manager about next season, they were confident about getting the right players," The Express quotes Mata as saying.
"They know, and I know, we're in a transitional moment, but in the summer they're going to try to sign a few players and I have the faith they are going to bring top players in."
United are currently seventh in the Premier League table, 10 points adrift of a Champions League spot.