The 24-year old, valued at £17.5million after his recent England debut, has attracted plenty of interest in the past year or so from the likes of Manchester United and Arsenal and Coyle knows that his departure is imminent.
"There will come a day, and I hope it doesn't come in January, that Gary Cahill will probably outgrow this football club given the finance involved in it. I firmly believe he could play for any of the top-four clubs," he said.
"And if that day comes, Gary would go with our blessing. He has been a terrific servant.
"The reason those clubs are in for him is because he has been playing consistently well for me as a manager.
"I have always said I have no qualms with anyone, as long as they pour their heart and soul into their job.
"And if that's the case, then he would go with my blessing."