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.