Federer announced last week that he had undergone a second surgical procedure on his knee which will keep him out of action until 2021.
The development concerns Gilbert, who feels the 38-year-old faces a real uphill task to return to his best.
“I’m not going to write him off, but this is concerning,” he told ESPN.
“Thirty-nine ain’t what it used to be, that’s for sure. But if tennis gets going again, he’ll probably be unseeded for the Australian Open in ’21 because he played so little early this year. And that could mean trouble.”
He continued: “We are in uncharted territory, the way guys are training and taking care of themselves.
“Sure, Ken Rosewall and Jimmy Connors [both played in majors after turning 39], but today’s game is more physical, and the players are tougher.
“When you take that much time, even at 25-26, it can take six months to get your mojo back.
Wimbledon 2001 winner Goran Ivanisevic weighed in on the debate and believes Novak Djokovic will overtake Federer in the Grand Slam haul.
“One day when everyone finishes their careers,” he told Nova TV. “Then we will be able to give an account.
“It’s difficult for me to give an answer because in my opinion Novak is the best and he is the only one who can win a calendar Grand Slam.
“If he’s healthy, I think he’ll break that record [of 20 Grand Slams]. I think Nadal will break Federer’s record as well.
World number 12 Borna Coric was asked by UBI Tennis last week: “Will Federer 2021 be a repeat of Federer 2017?”
He replied: “We have been saying he’s too old for 10 years. So until he says he stops playing tennis I will never count him out.”