Gaitan will become United's fifth signing this summer following the arrivals of Memphis Depay, Bastian Schweinsteiger, Morgan Schneiderlin and Matteo Darmian.
The 27-year-old has nine caps for Argentina and has been at Benfica since joining from Boca Juniors in 2010.
Gaitan has been linked with a move to Liverpool but according to the MetroUnited have agreed a deal to sign the winger.
And that could spell the end for Angel Di Maria.
GETTY
Nicolas Gaitan could move to Man United
Di Maria became United's record signing last summer in a deal worth £59.9million but struggled in the second half of the season.
The former Real Madrid man has been linked with a return to Spain and to PSG.
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