The France international penned a new deal until 2024 on Thursday, with the option to extend for a further year.
The news delighted former Old Trafford midfielder Scholes, who believes the 23-year-old is now getting a chance to express himself under Ole Gunnar Solskjaer.
"It's brilliant, he was another player who struggled under previous manager," Scholes told BBC Five Live.
"We all knew his qualities, I remember talking to (Dimitar) Berbatov who played at Monaco with him and he was raving.
"You need a run of games to express yourself and now he's getting that, he can beat a man, score goals, bring excitement for the fans and that's the most important thing. It's a great signing."
Martial joined United from Monaco for £36 million in 2015 and has scored 10 goals in 25 appearances in all competitions for the club this season – nine of those under Mourinho.
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