The Trotters have agreed a fee with Chelsea, reported to be around £7 million, for the transfer of Cahill, but personal terms have proved a stumbling block, with the England centre-back's wage demands on a five-year contract more than the Blues are willing to pay.
Newspaper reports on Thursday morning suggested that Chelsea manager Andre Villas-Boas is becoming frustrated with the impasse and the deal could be called off if terms are not agreed by the end of the week.
If a deal is not struck by Saturday, Coyle will be happy to call on Cahill for the trip to Old Trafford.
"Dialogue continues between Gary's agent and Chelsea," Coyle said. "We accept it has probably been long-drawn. It is over two weeks ago that we agreed a fee. But sometimes that is the nature of football business. It can take that period of time.
"Gary is a Bolton player and until I hear something different in terms of negotiations being concluded between both parties, he will be in line to play against Manchester United."
Cahill only has six months left to run on his Bolton contract and it has been suggested that rival suitors have made their interest in the defender known. However, this is news to Coyle, who insisted Chelsea remain the only current option for Cahill.
"If other clubs are linked with him it is just speculation," he said. "We have agreed a fee with Chelsea. I have had no dialogue with any other club.
"We recognise it is the silly season but I don't deal in rumour. I only deal in fact. The fact is we have agreed a fee with Chelsea. We have not spoken to anyone else. That is where we are."
Coyle did not confirm USA international Tim Ream was coming in for a medical. However, having had the New York Red Bulls star on trial, he is a fan.
"You could reel off 30 or 40 names that we have been linked with. That is the nature of the transfer window," Coyle said. "Tim Ream is a player we had in. He has been at West Brom. They were very keen on him as well and Arsenal tried to sign him in the summer.
"He is an outstanding young player. A US international, 24, and no doubt destined for a very bright future. Whether anything transpires with Bolton time will tell."
Coyle also said goalkeeper Jussi Jaaskelainen had an outside chance of being fit for the Old Trafford trip, although he will not be taking any chances with the long-serving Finn.
"Jussi is responding to treatment with his thigh knock, although Saturday may just come too soon."