The skilful winger has long been considered as one of Portuguese football’s finest talents and has been on the Red Devils’ radar for the last 18 months.
Gaitan starred for the Lisbon side as they ran to the Europa League Final and secured their domestic league title recently, which appears to have convinced United to launch a £20m move.
Reports suggest that Louis van Gaal is ready to have the fee paid in full and up front, as he plots to bulk out his squad ahead of a push to get the club back into the top four and potentially challenge for the title.
Flop winger Bebe may be included in the deal, with the much-maligned wide-man having thrived during a loan spell back in Portugal.
Benfica are said to be ready to sell their man as they look to sort out their finances, but they are a club known for their tough negotiation skills.
Despite his impressive displays during 2013/14, Gaitan did not make Argentina’s World Cup squad.