That’s the verdict of former England manager Sam Allardyce.
Lukaku has been heavily linked with a United exit this summer, with Inter Milan leading the race for his signature.
The Belgian arrived at Old Trafford two years ago in a deal worth £75m.
But despite two prolific campaigns, Lukaku has struggled for regular first-team football since Ole Gunnar Solskjaer was appointed.
The Norwegian preferred to use Marcus Rashford as a lone striker last term, with Lukaku’s minutes limited to appearances from the bench and on the wing.
The 26-year-old is expected to seal a move to Serie A, and United have been linked with a number of replacements, including Moussa Dembele and Wissam Ben Yedder.
Costa featured just 16 times in La Liga for Atletico Madrid last season and the former Chelsea star is attracting transfer interest.
And Allardyce admits he would target the Spain international if he was in Solskjaer’s position.
“I think he’d be a fit for United.”
Former Man Utd striker Alan Brazil
The 64-year-old was discussing Lukaku’s situation alongside Alan Brazil on talkSPORT.
“I think he’d be a fit for United if they got rid of Lukaku,” Brazil said of Costa.
“Lukaku by all accounts this morning, representatives of Inter Milan are talking to Manchester United.
“If you were manager of Manchester United, you would not sell him?”
Allardyce replied: “Lukaku? No.
“Not unless I was getting Costa.
“I might say yes to that.”
Inter are keen to structure a loan deal for Lukaku but reports claim United have made it clear they want to recoup the £75m fee they paid.