Rashford scored for the 10th time in 10 games since the World Cup.
Rashford scored for the 10th time in 10 games since the World Cup to put United on course for a comprehensive first-leg win at the City Ground.
The 25-year-old is arguably the in-form player in the country at the moment and produced a moment of individual brilliance as he ran 50 yards and then converted at the near post.
Keane was highlight critical of Rashford as he lost his place in the United side last year, but he believes his attitude on and off the pitch has been transformed.
"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," former United captain Keane told Sky Sports.
"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.
"Sometimes it's about confidence - there is where he's peaking now. He's played a lot of games for United and big games for England in the last few years.
"Sometimes the penny drops for a player in terms of decision-making and putting the ball in the back of the net. And they've needed him.
"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.
"The best players turn up week in and week out. It looks like he's now enjoying that responsibility, which is great to see."
Keane suggested maturity is a key factor in Rashford's improvement, as well as an unbroken run in the team.
"He's 25, not a child, sometimes when the timing is right you can be ready for that," added the Irishman.
"He doesn't get the niggly injuries anymore with his groin, he seems to have cleared it up. He looks in a great place mentally, too, which is huge. He said he reckons he took too much on away from football - and being a top footballer is hard enough.
"So putting the other stuff on the back burner and focusing on being a top player for Manchester United has seen him get his priorities right."
United boss Erik ten Hag suggested Rashford was unstoppable, as he continued his run of scoring in every round of the Carabao Cup.
"There are more players who are playing really well but I am happy with Marcus’ performance and his development," said the Dutch coach.
"From the start of the season he is growing and he keeps doing that and what we have to aim for with him, and he is responsible for that, is to keep this process going.
"If he is in this mood, this spirit, he is unstoppable and it’s about the team to get him in the right positions.
"Also, possession, we have a plan but finally is also the intuition, the creativity of an individual like Rashy is to create chances."