Bolton have signed Aston Villa defender Chris Herd on a one-month loan deal.
The Division Three leaders have already approached United about a possible deal and are also keeping a close eye on Motherwell midfielder Nicky Law and Middlesbrough defender Andre Bikey as they prepare for next season.
A transfer embargo means Rangers are restricted to targeting players who are out of contract in the summer and can only register free agents from September 1 under terms of the ban.
"We have to sign players whose contracts are up at the end of the season," he said.
"I'm very aware and respectful that these players are still with other clubs and I wouldn't want to do or say anything that would influence them in any way in their relationship with their own clubs.
"All I would say is that Jon is one of a number of players we are looking at and are interested in.
"There has been contact between the two clubs and there has been contact between club and agent but there is nothing more concrete to tell you on that one."
When asked about other targets, McCoist said: "Nicky is another player whose contract situation we are aware of as well.
"Again, he is one of a number of players we are looking at.
"Bikey is a player that we are interested in and have been interested in once or twice, certainly last year.
"He is a player we have been monitoring as well.
"I'm not sure if he's got a different situation with his contract but he is certainly a player that we have been watching regularly."