David Duval yesterday urged Rory McIlroy to take a long break for the game now so he can fully recover from his rib injury and avoid doing long-term damage to both his swing and his body.
The former Open champion and world No 1 is speaking from painful experience after a sprained fifth lumbar vertebra threw his back out in early 2000.
Duval lost his swing trying to compensate for the injury and never won again after 2001.
"He's hurt and I am watching his golf swing deteriorate," Duval said on Golf Channel at Quail Hollow.
"If only I could go back and tell myself 18-20 years go when I started having those problems, 'Stop, get healthy'. He could do himself a big service."
Duval won 11 of 34 tournaments from the end of 1997 to early 1999 but just three from 2000 to 2001 before crashing to 527th in the world by the end of 2004.
Concerned for McIlroy's swing, Duval said: "He's always had a little bit of a hitch with the driver in terms of flattening out a little but it is getting a lot more pronounced right now and I think that is due to that rib injury.
"That's the only thing that makes sense; not being healthy and not being able to rotate.
"At the top of his swing he is dropping it down a fair bit under the plane and that's why he is missing it right and missing it left because you have opened up the face, you have raised the handle of the golf club and you have to roll it and time it.
"And that's when you get it firing out of both barrels if you will, and it can go way right and way left. And I think he is doing himself a disservice by playing right now."