Manchester City midfielder Ilkay Gundogan is set to miss several months of action after suffering a bad knee injury during Wednesday's 2-0 win over Watford.
The German midfielder jarred his right knee in the first half at the Etihad Stadium and after unsuccessfully trying to play on, came off in clear distress just before the break.
Gundogan dislocated his right knee cap last spring and spent several months in recovery prior to joining City, whose boss Guardiola expects more bad news for the player.
"I'm so, so sad and unhappy for what happened to Ilkay Gundogan. We're going to miss him I think for a long time, it is the worst news from the day," he said.
Asked if the player was looking at a prolonged spell out of the side Guardiola said: "The doctor supposes yes but we will know tomorrow. I think so.
"Tomorrow we will know exactly the diagnosis. It looks like it will not be good.
"He is sad, so sad."
Gundogan's team-mate Pablo Zabaleta, who opened the scoring for his side with his first goal in two years, commiserated with the former Borussia Dortmund man.
"Yes it looks bad. Same knee," he said.
"He will have a scan and we will see how bad it is but we feel so sorry for him because it has been quite tough for him at the beginning of the season.
"He has been working so hard and he was playing well. Unfortunately it is the same knee but you have to keep strong and we will see how bad it is."