The reigning Scottish Premier League champions have spent much of the summer trying to put a deal in place for the Spaniard.
A price has been agreed with Aston Villa for the 29-year-old, who left Ibrox for Villa Park in 2008, but negotiations have stalled since then.
McCoist is hopeful an agreement will be reached, but admits he will be forced to turn his attention elsewhere if the matter is not resolved soon.
"There has been no news yet, unfortunately, from Carlos. He's not been in touch," the Gers boss told the Daily Record.
"We wanted an answer from Carlos today, definitely. He knows that, as does his agent and lawyer.
"They all know we are pressing for an answer as soon as possible.
"How would I assess our chances? I have no idea. There is only one man who can answer that and it's Carlos.
"I understand he's in Spain and I don't know if he's talking to other clubs there but I've never asked for any assurances."
McCoist is hoping to have Cuellar, and other potential new recruits, on board by the time the deadline for registering players for the UEFA Champions League passes on Thursday.
He said: "In an ideal world, I would love to have them all in by Thursday. That would be fantastic."