The Red Devils have already hit a blip in form this season, losing to Swansea and drawing to Newcastle at Old Trafford.
And former defender May believes Louis van Gaal must put aside his differences with De Gea if he wants to get back to winning ways.
United host Liverpool this Saturday (5.30pm) in what is traditionally their biggest clash of the Premier League calendar, and May predicts they'll need the Spaniard between the sticks.
He told talkSPORT: "Now he’s going to be here until next season then there’s no reason why not.
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David De Gea has been urged to start against Liverpool
“You’ve got to play your best players and David De Gea is by far the best goalkeeper at Manchester United.
“He’s the number one goalkeeper in the Premier League so why on earth would he leave him on the bench or in the stands?”
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