Speaking to the Daily Mail, Giggs, 51, admitted he would be surprised if the 27-year-old plays for Man United again having joined Aston Villa on loan in January.
Giggs said: "I worked with Marcus and I remember going over with Louis to watch him train with the reserves.
“It was one of those moments where you just can't leave him out of the team and he was a breath of fresh air.”
The former United winger acknowledged that Rashford has struggled to deal with the pressure at the 20-time English champions.
He added: "Looking from the outside, it does look like he's got the world on his shoulders and he's forgotten what it's like to play with freedom. It's great to see him doing well at Villa. I don't know whether he'll come back."
Rashford is currently away with the England national team having been recalled by new manager Thomas Tuchel, making his first appearance since their 1-0 friendly defeat to Brazil in March 2024 in the World Cup qualifier against Albania.