The United loanee struck both goals as Barca won their Champions League opener 2-1 at Newcastle on Thursday.
Butt told BBC Sport this morning: "Marcus showed what a top, top player he is. We forgot about his talent at Manchester United but he showed he is a real top talent and back to the old Marcus.
"His work-rate as well, he was very positive in what he was doing. From a young boy he has always threatened a goal and the goal was an amazing header.
"It was an unbelievable strike (for the second goal) and he has got that in his locker. He is a great striker of the ball and he was the best player.
"Ruben Amorim will be thinking 'what have I done, I've let that go' when Manchester United are struggling. You need to be loved and he needs an arm around him and when he has that he has shown what he can do."
Opening up
Meanwhile, Spanish expert Guillem Balague also commented: "All Barcelona fans heard was about the time at Manchester United where people seemed to still celebrate, based on social media this is, the fact that he wasn't scoring, that he wasn't at his best at Barcelona, that he hasn't shown the level that he's got.
"I don't know why Man Utd fans are rejoicing when he doesn't succeed because they've still got to buy him, he's on loan.
"But in any case what Barcelona coaches had seen was a shy guy. It has been difficult to get through to him as he was somebody that's been hurt and wouldn't open up very easily.
"He didn't have the kind of passion or whatever it is that Raphinha or Lamine have when they don't start the game but as Hansi Flick has said, they've seen his quality. And the interesting thing is they see him more of a nine.
"They see that because he shows flashes of his ability on the left-hand side with the ball but without the ball he tends to forget that he has all this stuff to do and that's no good because that wing becomes very easy to attack."