Manchester City have confirmed winger Leroy Sane suffered ankle ligament damage during the FA Cup fourth-round win over Cardiff.
The Germany international was substituted at half-time following a challenge from Cardiff player Joe Bennett.
City say scans confirm the 22-year-old has ligament damage and is set for a spell on the sidelines.
The club has not put a timeline for his return.
Bennett was booked for the tackle on Sane and then sent off after picking up a second yellow card later on.
Guardiola later called for referees to do more to protect players from bad challenges.
He said: “The only thing they can do is that – protect the players, because then it would not happen again.
“It happened once with Leroy, then it happened with Brahim again at the end.”
Cardiff boss Neil Warnock defended his players, saying it was “part and parcel of the game.”
City went on to win the FA Cup match at Cardiff 2-0 thanks to goals from Kevin De Bruyne and Raheem Sterling.
Sane has scored 11 goals and produced 14 assists in all competitions this season.