Cahill left Bolton Wanderers for the Blues in January in a deal worth to be around £7 million, with the two clubs coming up against each other this weekend.
However, the 26-year-old has made just one Premier League appearance for his new club following his protracted move to Stamford Bridge, while his start in the 3-1 defeat to Napoli in midweek was just his third since moving to West London.
But the Scot insists that the powerful centre-back will enjoy a glittering career for both club and country despite an indifferent start to life with Chelsea.
"Gary is an outstanding player, who is conscientious, who cares and who doesn't like losing at any time," Coyle said.
"There is no doubt that he will have a terrific career at Chelsea.
"It is always difficult in the January window when players are thrust right into it. I think Chelsea were clever in giving him a couple of weeks' training with the players before he stepped into the team.
"The team has had some indifferent results and Gary is part of that, but there is no doubt that he will achieve big things at Chelsea."
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