The Scot is expected to be sidelined until the end of the winter with a fracture in his foot and will undergo an operation next week.
It is the latest issue to befall the former Blackpool player.
Earlier this year, Robertson suffered an injury in the same foot during rehabilitation from stomach surgery.
Warne, who is confident that Jamal Blackman will be fit for Saturday's game at Reading after picking up a knock in midweek, said: "It's a massive blow, on top of the fact we have missed Chieo (Chiedozie Ogbene), they are two big players for us.
"I think in the games he has played in this season, he has been good for us, so it is disappointing for Robbo, and a big blow for the team.
"I feel for the kid, he has been in good shape this year and he has just been blighted by an innocuous challenge that has done severe damage. It is really hard for him and a big blow for everyone.
"It can happen all the time, we are in a contact sport, but it is just a pity that we probably won't see him in a game for the next four months.
"It is heart-breaking for him, especially after last year as well. It is a difficult time, but we are here to support him and he knows we are here for him.
"That is the sad part of professional football when you are out injured and all you want to do play and you can't, it's tough for him.
"He will get the arm around the shoulder, even if he doesn't want to in these social distancing days.
"I think this will hit him the hardest because he had similar sort of situation last year and he was out for a period of time. He got himself in good physical nick and from a psychological point of view it will hit him harder than the previous injury.
"It is not a career threatening injury, but it is a blow and he will get our full support."