3 years shouldn't matter much when you're 23-24 at the start. You don't experience much physical drop off due to age in that time. If the knee is healthy, his physical condition should be fine, and he won't have lost his tennis skills.
And when I see the beatdown he put on everyone just last season, I don't worry too much.
Check the mileage on that engine though. Also check the wear and tear on the body. Not saying he is 'done' by any means. Obviously not. But he is becoming more and more beatable even on preferred surfaces, a quarter-step slower, and more and more prone to injuries, more vulnerable Rosol-Zeballos-like 'surprises', etc.
Nadal was an early bloomer. He won a slam at 18. Tennis prime is usually 21-27, more realistically 22-26 for most. Even Federer started to taper off around 2008. Federer happens to be a longevity freak but, even so, look at his results. Great for a geriatric tennis player, but obviously he is not playing at 2004-2007 levels and never will again. Same happens to everyone. Time is relentless and especially brutal on minds and bodies in a sport like tennis.