The Portuguese defender has been out of action since mid-February.
"Ruben can perfectly handle the lack of time playing for seven weeks," said Guardiola.
"He can handle it perfectly. The other ones need step-by-step minutes.
"I think for seven weeks he went to bed at 10pm up at 7am, lunchtime at 3pm every day.
"He was in the gym and the pool. That's why in his mind he says he is OK.
"I would say it doesn't depends on the position (when he is ready), it depends on the player."