John Stones' injury in the Champions League victory over Young Boys will be a "big loss" for Manchester City, says manager Pep Guardiola.
The versatile England defender, 29, was replaced by Nathan Ake at half-time, with Guardiola telling TNT Sports "I feel so sorry for him".
"He is so important for what he does," added Guardiola.
Stones missed the start of the season with a hip injury and only returned to action in October.
When asked if he expects Stones to be kept out for a while, City's Spanish boss added: "I think so. [It is a] muscular injury, it will be a while.
"It is a pity, it is bad news for us. I don't know how long."