"That is completely out of order," said Carlisle.
"Terrell's such a good player. He's covered for me during games so many times and I would not want to be without him."
Forbes, 22, was a central defender prior to being released by West Ham in the summer of 2001, when he moved to Loftus Road.
But he has operated mainly at right-back during his time with QPR.
"Some people are maybe forgetting that he's still young," Carlisle added.
"His first season with us was his first season in league football and he was outstanding.
"He's always getting other defenders out of trouble and is a vital player for us."