The Italian striker, 21, is certain to leave Old Trafford on loan during January with his agent having admitted that the option of a stint in Spain is interesting to all parties.
The former Lazio youth teamer does not want to go back to Italy after an unhappy experience at Sampdoria two years ago when the club were relegated amid allegations over match-fixing in the camp.
Macheda is desperate to play regular first-team football after making only two starts and eight substitute appearances in the Premier League for United in the past four seasons.
His last league goal for the Reds was at Aston Villa over two years ago and he is unlikely to be benefiting from having to get the vast majority of his football in the development league.
United might be happy to see him go to Rayo with the attacking philosophy of their coach Paco Jemez currently taking the club into Europa League contention.
Rumours that Rayo are ready to sell their Brazilian frontman Leo Baptistao could also boost Macheda's chances if a deal can be done before the end of January.
