Portuguese newspaper A Bola claim long-term target Gaitan has said goodbye to his Benfica team-mates after agreeing a move to United.
The 27-year-old has been on the Red Devils radar for years with Sir Alex Ferguson and David Moyes both interested.
But it's Louis van Gaal who is reportedly the man poised to seal the deal.
A Bola claim Gaitan's agent Jorge Mendes is on his to Manchester to finalise a £28m move this week.
Mendes represents a host of top stars including Angel Di Maria and Cristiano Ronaldo. He played a crucial role in bringing Radamel Falcao to Old Trafford last summer.
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United have cash to spend following the £45m sale of Di Maria to Paris St-Germain last week.
They are also interested in Barcelona hitman Pedro, 28, with a move set to be sealed tomorrow.
Gaitan, who has a £35m release clause in his contract, is a tricky winger who prefers playing on the left.
He caught the eye of ex-boss Ferguson in a Champions League game back in 2011.
Fergie, speaking after the 2-2 draw, said: "Gaitan played with intelligence and showed great individual skill."