Mendy has damaged knee ligaments just weeks after joining City from Monaco in a £50m deal.
The left back is now set to fly to Barcelona to see specialists and undergo scans to discover how long he will be out for.
Pep Guardiola admits he fears the worst for the French international, who could be sidelined for several months at least.
Gundogan knows what his team-mate is facing up to, having just spent nine months on the sidelines himself with serious damage to his knee ligaments.
The German international has spoken to Mendy and insists Guardiola's squad will be there to support him.
He said: "Of course I spoke to him but, not really about the situation.
"I think he's a guy who's the most disappointed one. I think our task now is just to support him and just to be there if he needs something from us.
"I know how difficult these kind of situations are. There's not really much to talk about at the moment.
"We just should try to behave as normal as possible. We wish him a very speedy recovery and we will support him in every way."
