Mendes has previously helped United bring the likes of Radamel Falcao and Angel di Maria to Old Trafford.
Portuguese newspaper O Jogo reports that while Mendes does not represent Gaitan he has assumed a negotiator role to work on a move.
Gaitan has turned down a move to United Arab Emirates side Al Ahli as he is keen on a move to the Premier League.
The Argentine has a £24.5million release clause in his contract at Benfica and is also being linked with the likes of Atletico Madrid and Valencia.
