Which Roger Federer will we see when he comes back?

— -- Roger Federer's injury-plagued season took a turn for the worse Tuesday, when he announced he will take the rest of the season off to further rehab his ailing left knee.

At 34 years old, what does this mean for the game's all-time Grand Slam winner?

Our experts weigh in:

How much of a surprise was the announcement that Federer is taking the rest of the year off?

Pam Shriver, ESPN tennis analyst: Not an extreme one. Given what we saw at Wimbledon, the way his legs seemed to give out in the [Milos] Raonic match and how fast he came back from knee surgery earlier in the year, it's not all that stunning.

Greg Garber, ESPN.com: Not a ton, actually. Since the knee surgery back in February, he's been at-risk. When he passed on a few spring tournaments, the word was his back was the culprit. Now, it seems to have been the knee all along.

Melissa Isaacson, ESPN.com: A surprise, but not a shock. He looked strong enough at Wimbledon but made it clear he was not 100 percent. At 34, even for Federer, this was always going to be a possibility considering his injury-plagued season.

Brad Gilbert, ESPN tennis analyst: When he first injured his knee after the Aussie Open, we weren't sure of the extent. But I think we are all surprised; he must have received some medical news that he had to shut down his season. As much as it is a surprise for us, he's been nearly indestructible in his career, and that has to be a shock to him.

Jim Caple, ESPN.com: Not incredibly surprising given he has missed so much already, including the French Open. Even so, it's a little bit surprising considering that he wanted to play in the Olympics and that the US Open still is coming up.

Nic Atkin, ESPN UK: As someone who has twice had the same meniscus knee surgery as Federer, I can tell you for certain it hurts like hell to fall on it as he did at Wimbledon against Milos Raonic. It can also tear again easily, and Federer is being wise to rehab it and not rush back.

Peter Bodo, ESPN.com: It was probably less of a surprise to those of us who were at Wimbledon, where there was a lot of talk about the state of Federer's knee. Still, this did come somewhat out of the blue.

At 34, how much does this setback have to hurt at this point in his career?

Shriver: He has a special ability to be in total acceptance of his fate. He has an extreme gift when it comes to footwork and understanding the game, which will help him when he comes back. So that has to give him some solace.

Garber: More than you can imagine. Federer, in his mind, is invincible. This is his first serious injury, which forced him to miss his first Grand Slam event after a record 65 straight.

Isaacson: It was clear in his statement that this bothers him, but he also is smart enough to weigh the alternatives. Sacrificing the rest of his career to play the Olympics and the US Open did not make sense to him.

Gilbert: It's not like he's 26 or 27 anymore and can come back with few worries. And who knows if he'll need more surgery down the road on his knee. There are only so many setbacks he'll be able to endure at this point in his career.

Caple: This cannot have been an easy year for Fed. He'll have missed two Slams, and he not only did not win a tournament, but he didn't even make it to a final after hurting his knee after the Aussie.

Atkin: Federer is trying to be as upbeat as possible about this, but his body is giving up on him. He rebounded well from his 2013 struggles and came close to an 18th Grand Slam title, reaching the Wimbledon finals in 2014 and '15 and the US Open last year.

Bodo: I think his hurt was less about his age and upcoming missed opportunities than the simple disappointment of missing something he was really looking forward to. It was sort of like having to cancel a dream vacation, which is how Federer views tennis and why he is who he is.

Presumably, Federer will never win an Olympic gold in singles. How does this affect his legacy?

Shriver: Won't affect his legacy at all. He won gold in doubles, which was extremely important to him. And given everything else he's done in this game, this won't matter at all when looking back at his career accomplishments.

Garber: Within the tennis community, I don't think it makes any difference. Hey, he does have that gold from Beijing playing doubles with Stan Wawrinka.

Isaacson: Not a wit. The Olympics are a nice asterisk in a tennis player's career, and Federer already has a gold in doubles and a silver in singles. If Serena had not won a single gold medal, would we think any less of her? Don't think so.

Gilbert: Well, he has an Olympic gold in doubles. Aside from Steffi Graf, Andre Agassi and Serena Williams, there are no players who you can point to who have won every major event. No one will ever look back at Federer's career and say, "If only he had an Olympic gold medal in singles." He's done it all aside from that.

Caple: Not at all. While he certainly wanted a gold medal in his collection, most people won't know -- or care. They'll just concentrate on his 17 Grand Slams, his many other wins and his fame.

Atkin: It's the one major honor Federer has never won, but that won't keep him awake at night. As Rory McIlroy said earlier this month, golfers grow up dreaming of green jackets and Claret Jugs, not Olympic gold medals. For tennis players, it is all about Grand Slam titles.

Bodo: I can't think of a single item that would impact his resume less than this Olympic shortcomings. The vagaries of the quadrennial competition, the fact that he has medaled in two of the three disciplines, all weigh in his favor and make the lack of a gold more anomaly than demerit.

What do you expect from Federer next season at the age of 35?

Shriver: It'll take a few tournaments, but after a couple of months, I think he'll be back at a really high level, the same level he was at this year when he was playing his best tennis.

Garber: More of the same, provided the knee recovers. He's still better than 98 percent of the professional players out there. But losses to young guys like Alexander Zverev and Dominic Thiem will be more commonplace.

Isaacson: If he comes back strong, there is no reason to think he can't put together a few nice runs in Grand Slams. With a healthy knee, maybe he'll even win a tour title, but I wouldn't expect a complete rebirth at 35.

Gilbert: We have to remember, he looked great at Wimbledon. But you can't do anything about Father Time, especially in sports. More than anything, it's going to be depend on whether he loses a step after being out so long.

Caple: I don't expect much considering he hasn't won a major since 2012. Not many tennis players win at that age, especially when they need to overcome a serious knee injury. I wouldn't be surprised to see him retire by the end of the season.

Atkin: It is hard to see Federer challenging for majors ever again, let alone next season. His intense drive, professionalism and will to compete mean he will try with all his might to prove that assertion wrong, but time simply isn't on his side.

Bodo: I expect Federer to play a highly selective schedule, partly because of his knee. And I expect him to have highly mixed results -- great and mediocre.