Can't see why the players would need to land near to Old Trafford for the most part. - They all live pretty closeby.
Maybe to land at the training ground at Carrington, but not to get to OT itself.
However, I'm sure Barton is the most likely place. It's pretty congested in terms of land when you start getting closer to Salford, unless you can use an empty MUFC carpark.
It's only a 10 minute drive from Barton on a non-match day, and I can't see why players would need to heli in to OT on match days.