The Frenchman was wanted by Jose Mourinho at Old Trafford this summer. However, the Court of Arbitration for Sport's (CAS) decision to uphold the Spanish capital club's transfer ban saw Atletico convince Griezmann to continue at the club. He committed his future by signing a new deal.
Greizmann is comfortable playing anywhere in the front three. Rashford, who started his career in the central position under Louis van Gaal in the second half of the 2015/16 season was moved out to the wings after Jose Mourinho arrived at Old Trafford last summer. However, an injury to Zlatan Ibrahimovic in April saw the England international play as the main striker.
McGuinness has also compared the 19-year-old to former United star Cristiano Ronaldo and explained what he needed to do to improve his goalscoring record next season.
"He's [Rashford] a really good footballer, a modern footballer, he can play anywhere across the front," he said. No one complained about (Cristiano) Ronaldo playing wide," McGuinness told the Manchester Evening News.
"He is a player who can adapt. Eric Cantona didn't just play up front. You need people to move around."
"Griezmann drops and plays as a No 10. The footballer of the future can interchange positions - Marcus can do that.
"Ronaldo wasn't a top goal-scorer to begin with - he was full of tricks. It just seemed to happen overnight - that realisation that you get all of the attention when you score all the goals. Now he is the ultimate centre forward.
"Marcus can score all sorts of goals, but to be a goal-scorer you need more of the Ruud van Nistelrooy type of goals. If it hit the goalkeeper - there was Ruud. If it hit the bar, post, deflected - he was there. Marcus has got a bigger range, but he needs more of those."
McGuinness revealed Rashford wanted to play like Italy's midfield legend Andrea Pirlo and it was a collective decision between himself, Warren Joyce and Colin Little to move the teenager from the midfield to the attacking position.
"We decided, myself, Warren Joyce and Colin Little, that he was a very good footballer. He could be a No10, a winger, beat people, he used to drop off into midfield. There was one tournament where he wanted to play like Pirlo. But we saw his potential to be a striker," he said.