Rafael Nadal was almost out for a year after suffering tendinitis in his knee, which subsequently led to him pulling out of the US Open and the 2012 Olympics.
The Spaniard has been in fine form since returning to action. He has taken his Grand Slam count to 13 this year after winning the French Open - his eighth - and a third US Open title.
Nadal is getting closer to Roger Federer's all-time record of 17 Grand Slams, but failed to make it past the first round at Wimbledon loosing too Belgian Steve Darcis.
Agassi, who won eight Grand Slams between 1992 and 2003, believes Rafa does not have to win any more titles to be considered the best of all time.
Agassi, who was speaking at the UK launch of his athletics equipment, told reporters: "I think it's fair to say that Rafa has a shot at it (passing Federer) and one might argue has a good shot at it.
"I don't see anyone winning the French (Open) for the next couple of years, but him. But that's assuming he stays healthy.
"But this guy I don't think has to win more to prove that he can be argued as the best of all time."
Andy Murray famously became the first British winner of Wimbledon since Fred Perry this summer, but lost his US Open title as he fell in the quarter-finals to Stanislas Wawrinka.
Agassi, who won his first Grand Slam at Wimbledon, insists he has not been surprised to the Scot slip away after his incredible performance in the summer.
"I expected a let-down," said the 43-year-old. "I've believed in him for years. When I watch what he can do on the tennis court, I say he's as good as anybody at this best.
"But it's also when he's playing with conviction, it's also when he's playing with being focused and he's not distracting himself.
"There's a lot that goes into winning these things and a lot of it's in your head and a lot of it's preparing right.
"He's starting to figure all those things out and he has a lot more to come."
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