The 25-year-old has been linked with a number of clubs over the summer, including Arsenal and Liverpool.
"I have said to Gary he can improve here with Bolton, and he has improved," said Coyle. "In the 18 months I have worked with him, Gary has become an integral part of the England squad because of his performances and has improved no end.
"There is no doubt he has taken his game to another level, but that was always going to happen with his age as well.
"Hopefully with the platform we can give him and encouragement as a staff we can help him on his way to a fantastic career."
Cahill scored the opening goal in Bolton's 4-0 win against QPR on the opening day of the season.