Bolton have agreed a fee with Chelsea for their England defender and it was anticipated the transfer would have gone through early this week.
However, personal terms have proved far more difficult to sort out than most observers anticipated and as the clock ticks down to the weekend fixtures, Cahill remains a Bolton player amid rumours Chelsea manager Andre Villas-Boas is getting increasingly frustrated.
"Dialogue continues between Gary's agent and Chelsea," said Coyle.
"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."
It has been suggested rival suitors have made their interest in Cahill 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," said Coyle.
"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."
