The Portugal international missed all five matches in November with a calf problem but returned at the start of December and has been an ever-present this month.
But he will not be available for City's trip to Aston Villa on Saturday, while there is uncertainty about the fitness of goalkeeper Ederson.
"Ruben is out for a long time and Ederson I don't know if he will be ready tomorrow," Guardiola said on Friday.
The City boss added that Dias would be out for "three or four weeks".
"After 75 minutes against United he felt something," he said. "He's strong and wanted to stay on the pitch but now he's injured."
Injury-hit City have suffered a dramatic collapse in form over recent weeks, losing eight of their past 11 games in all competitions and are fifth in the Premier League table.
Nathan Ake remains out but fellow defenders Manuel Akanji and John Stones are back in full training.
"Manuel and John are back in training," said Guardiola. "That is good for us. As many players that are back the better.
"I don't know yet (if they can start against Aston Villa). Yesterday was their first training session, I would say, properly. They have been out for a while."
