Stones previously said in an interview that injuries had caused him to suffer, and he even considered retiring last season. Rooney said he was shocked.
Rooney said: "It's surprising to hear Stones say that, but constant injuries do have that effect on players. It's tough because you're eager to play but you see your teammates training every day and playing in important games. It does affect you. The mindset, it makes you start thinking about other things.
"I've been lucky enough to never have a serious injury, but I've seen some injured players who spent every day rehabbing in the gym while their teammates trained on the field, and it can be really stressful. "