According to Former Manchester United chief scout Mick Brown, United are desperate to get rid on Onana following a disastrous two-year spell at the club.
Ruben Amorim is hoping to rebuild the Man United squad in his image following the club’s worst ever Premier League season, finishing in 15th.
He is said to be a big fan of Martinez, 32, with the World Cup winner increasingly likely to leave Aston Villa this summer.
Speaking to Football Insider, Brown said: "Man United have made it clear they would like to sign Martinez. They’ll take whatever they can get for Onana, and then they’ll make contact with Aston Villa and see if they can get a deal over the line.
“He’s seen as a huge upgrade on Onana so they’re prepared to take a hit to get things moving.
"If Man United could swap Onana for Martinez tomorrow, they’d jump at it. Martinez is in his 30s now, but he’s a very capable goalkeeper and they’re not concerned about that.
“The only issue, as things stand, is that they need to move Onana away before they go for Martinez because they don’t want to end up with both on the books.
“They’re waiting for a bid to arrive so that they can accelerate things and start speaking to Villa. For now, it’s just what they would like to do, not something they’re able to do."