Robertson, a summer arrival from Blackpool, has been hindered by a troublesome groin problem and will now be on the sidelines for the forseeable future.
Warne, whose side visit Aston Villa on Tuesday night, said: “Clark is still out for a long period.
“Unfortunately, he has a problem with his groin. He played through it last year, I think.
“Now he has had the same problem and it has made his comeback take a bit longer.
“I am obsessed with people getting injured. So we definitely won’t rush him back. We want him back quickly obviously, but we will not rush it.
“He will be out for a number of weeks, I am sure.”
Warne reports no other fresh issues, with Darren Potter (Achilles) still remaining sidelined.