The Spaniard only became the Red Devils’ record signing in January as he completed a £37.1m switch to Old Trafford from Chelsea, and enjoyed a good end to a disappointing 2013/14 fir the Red Devils.
However his future at the club is now the subject of some doubt, with Barca readying a £40m offer as they plot their own summer overhaul.
The silky playmaker is seen as an ideal arrival by Nou Camp chiefs, and it is thought that van Gaal is ready to consider big offers as he plans to rebuild United on the back of a woeful year.
The Dutchman is said to have his eye on playing Wayne Rooney just off of Robin van Persie, and with Mata most comfortable in that position his presence is not seen as vital.
A big money sale may also free up funds to add big name players such as Marco Reus and Yevhen Konoplyanka, with van Gaal thought to be keen on the duo.