Cahill's England cap has been a reward for the form he has shown at club level, but it could take a bid of as much as £17.5million to take him off Bolton's hands.
His performances have led to interest from several big clubs and Coyle conceded that, despite being a key man, if Cahill wanted to move on to bigger and better things he would not stand in his way.
"There will come a day, and I hope it doesn't come in January, that Gary Cahill will probably outgrow the football club, given the finance involved in it," he told the Bolton News.
"I firmly believe that he could play for any of the top four clubs. And if that day comes, then Gary would go with our blessing because he's been a terrific servant for the football club.
"The reason those clubs are in for him is because he's 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.
"He's different class when he does the simple things well and I think he'll go on to have a fantastic international career."