The France international moved to Manchester United from AS Monaco in a £36m deal under Louis van Gaal in the 2015 summer transfer window.
Martial scored 18 times in all competitions during his debut season at Old Trafford to underline his incredible potential.
The 22-year-old has struggled to kick on since Jose Mourinho took over the reins from Van Gaal at the 20-time English champions in May 2016.
Martial has been used largely as an impact substitute to fuel speculation that the France international could be on the move in the summer.
The France striker has netted nine goals and has made five assists in 26 appearances in the Premier League in the current campaign.
Former Manchester United defender May believes Martial needs to have more self belief to shine in the Red Devils team.
“It’s frustrating with Martial because on his day he’s unplayable but then you also have other days when you wonder if he’s bothered,” May told 888sport.
“You also have the addition of Rashford playing there and Pogba favours that left-hand side too and then you have Martial and Sanchez so it’s finding that right blend to helping them all play together. Then you have Ash too who likes to bomb down there.
“He’s still only a young kid and can destroy defenders but at times I think he needs to believe more in himself. He seems a confident lad but you never know what a 22 year old is thinking.”
Manchester United are in second place in the Premier League table with eight games to go in the final months of the 2017-18 season.
The Red Devils haven’t won the Premier League title since 2013 when Sir Alex Ferguson was still in charge at Old Trafford.
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