The England international, who missed the Trotters' 3-0 defeat at Manchester United, is expected to sign a deal at Stamford Bridge in the next 48 hours after agreeing personal terms.
"Gary [Cahill] was obviously in my plans before today but between the relevant parties they moved closer to the agreement they needed to make," Coyle said.
"Gary travelled down to London yesterday for a medical, which I believe has been concluded. You never want to lose a player of that class but we wish him the best."
Saturday's defeat at Old Trafford has left Bolton 19th in the Premier League table.