Premier League giants United signed the 19-year-old forward from AS Monaco on transfer deadline day.
BBC Sport has also claimed that the transfer fee for Martial could rise to £58m due to bonuses and add-ons.
Former Arsenal striker Henry is a fan of his French compatriot, but he has advised the United fans to be patient with the youngster.
“I have spoken to him a few times, the last time coming when Monaco played at Arsenal,” the former France international wrote in The Sun. “He is a nice lad, from a decent family, and his attitude is good…
“I can see why people are comparing us. He certainly has pace to die for and that is such an important asset for any attacking player.”
“United fans have been patient over these last two years, when they have won nothing, and I urge them to be patient with this young guy,” added Henry.
Martial is a talented young player and is a bright prospect for the future, and United fans will have to be patient with him.
One should not expect the 19-year-old to lead the line for the Red Devils rightaway, but at the same time the Frenchman should try to hit the ground running.