Ryan Giggs has warned Anthony Martial he will have to work extra hard if he is to keep his place in Ole Gunnar Solskjaer's starting XI following the emergence of Mason Greenwood.

Greenwood scored his seventh goal of the season against Everton on Sunday making him United's second highest scorer this campaign and with the teenager making an impact Martial has been urged to up his game.

"He is talented, and he’s not a bad kid, but the problem he has is that he looks lethargic all the time. He looks casual, as if he’s not bothered," Giggs told Premier League Productions.

"When it comes off, and he’s doing brilliant things, you say it just comes naturally. But when it’s not, you think: he’s not running, he’s not trying, he’s not working hard enough, he’s not holding the ball up.

"So in my eyes he has to try extra hard because of his persona, because of the way that he moves. But he doesn’t look like he breaks sweat and when United are chasing a game you want someone to get across the front post – you might not score, but your teammate might score.

"It’s all these sort of things that you’re not getting off him at the moment. With Mason coming on and doing that, getting in positions to score goals, he [Martial] is going to be under pressure, because he’s not giving you anything defensively and he’s not giving you anything offensively either."

Greenwood has started just once in the league so far this season with five of his seven senior starts this campaign coming in the Europa League. But, Solskjaer admitted it is getting more and more difficult to leave the 18-year-old out given his form and Giggs believes it won't be long before he replaces Martial in the starting XI.

"He looks a threat, every time he comes on. I think Ole is handling him in the right way at the moment, bringing him on, impact player," Giggs continued.

"It’s just how long you can keep him like that, because also when he starts games he’s scoring goals.

"We talk about Martial, he’s not doing enough, he’s not much of a goal threat, no chances in the box, so it won’t be long before Mason Greenwood starts."