Rashford got United's 3-0 win underway over Forest in the first-leg of their Carabao Cup semi-final with a stunning solo strike in the sixth minute.
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The England international weaved in from the halfway line to breeze past Joe Worrall and Remo Freuler before placing a left-footed finish past Wayne Hennessey to score his tenth goal in ten matches.
After the match, Ten Hag gushed over the 25-year-old, telling Sky Sports: "In this mood and with this spirit, he is unstoppable."
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Speaking to talkSPORT, Ten Hag added: "He's infesting and the team's infesting. I think we are improving and our attacking game is improving as well. We create more and more, especially for offensive players.
"I'm really happy that they are creating more but it's because the team is playing better. We controlled the game, there are more movements and that's why we can bring people like Marcus Rashford into positions where they can score."
In the Sky Sports studio, United legend Keane admitted he has been pleasantly surprised by Rashford's recent form and praised his transformation into a fit and hungry striker.
Keane said: "The question mark I've had over Marcus in the last few years is, has he got that personality to step up to the plate? Well now it looks like he has.
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"We were pitch side when the players came out and he looked lean. You wouldn't want to be up against him. He looks like a lean, fighting machine.
"United have had some great strikers over the years, and I think they need someone like that. Marcus has to say he wants that responsibility to be the main man."
Wout Weghorst scored the first goal of his United career just before half-time while Bruno Fernandes netted late on to all-but end the cup tie before the second leg in a comprehensive victory for the Red Devils.
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But Ten Hag still pinpointed some problems and warned his players that the tie is not finished yet, urging them to cut out any complacency when they meet Forest next week at Old Trafford.
The Dutchman added: "We almost let them come into the game and we can't allow an opponent to get back into the game when we started so good.
"We dictated it totally then in one moment it can change. We need to avoid that.
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"I think in the second half we were much more composed and we tracked them. It was much better."
There was a moment of concern when Rashford looked to have taken a knock and came off but Ten Hag insisted that the substitution was planned with a crunch Europa League tie against Barcelona coming in February.
The forward has now scored 18 times in all competitions this season, a huge improvement on last year when he netted just five all campaign and talkSPORT host Jason Cundy was full of praise for Ten Hag and his coaching impact.
Cundy said on The Sports Bar: "This season [Rashford has] come back from the World Cup with a point to prove.
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"He's enjoying the role he's got and he's got a manager that trusts him and believes him.
"He's now playing on the left-hand side, Weghorst is down the middle… and you can see they've worked with him on that left.
"I feel he's been mismanaged by that club and there have been managers and coaches who have not got the best out of him.
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"Who would've thunk it? All of a sudden Ten Hag comes in and you've got a brilliant coach getting a brilliant performance out of him."
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