But Coyle has remained optimistic about Cahill's prospects of playing again this season and on Friday confirmed the 24-year-old could be back by the end of March.
"We did the scan yesterday (Thursday) and Gary will now go another two weeks without any contact but from that point he will be able to come back and join in with the group," Coyle said.
"I'm envisaging, fingers crossed without getting too far ahead of ourselves, that possibly Gary could be back within four weeks.
"Ideally I'd like him back earlier but we have to make sure Gary is fine.
"A specialist in the field has looked at him and there was a little bit of his rib when he turned that was catching on the blood flow to the vein, which caused the clot.
"Gary had an operation to remove that little bit of rib and the blood is now free-flowing, the clot is totally dissolved and the vein is in perfect order.
"The reason for him to have a couple of weeks' rest now is because of the operation and making sure that everything in that area is sealed. But he should be up and running and ready to go from there, which is great."
Coyle reaffirmed his belief that Cahill could still make Fabio Capello's England squad for the World Cup.
"We are delighted that he is going to be back and available, one for Gary, two as a boost for the football club and me as a manager, and three, I would suggest, for the country," Coyle said.
"I have no doubt that if he gets back to the level he is capable of - without putting too much pressure on him - then I think he will be a real candidate for the World Cup."
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