The Ibrox boss has informed midfielder Sam Kelly that a deal won't be on the table, despite the former Everton youngster impressing during a trial at Murray Park.
Boris Pandza and former Hearts star Marius Zaliukas, who has been training with the champions in recent days, are both on McCoist's radar.
And the Gers gaffer will have to choose one or the other after splashing the cash on eight new faces already this summer.
He said: "I've obviously made it clear we'd love to bring in a centre-half and Marius has done really, really well.
"He says he hasn't done an awful lot lately but you wouldn't know it to look at him because he is certainly in good shape.
"Marius has come in to train with us and is a very fit lad.
"Obviously himself and Boris are two players we've been looking at and I'm very keen to strengthen that department.
"I'm not sure if there's a budget for both and I would need to speak to Craig (Mather) about that.
"He's very aware of our concerns about getting another centre-back in.
"We couldn't afford to offer Sam a deal so that would possibly indicate we'd be looking at getting one centre-half in."
While McCoist is still eager to bring another player in, two members of his squad could be heading for the Ibrox exit door this week.
Scott Gallacher is in talks with Airdrieonians while Andy Mitchell is attracting interest from Arbroath and Annan.
"I've spoken to Jimmy Boyle at Airdrie and I know Scott spoke to him on Friday as well about going there," McCoist told rangers.co.uk
"Hopefully we're going to get something over the line there in the next couple of days, where Scott can still train here and continue his work with Jim Stewart and Jim Henry.
"At the same time, he'll train with Airdrie when they meet and obviously play with them as well.
"That way, we feel we'd be getting the best of both worlds. It ticks all the boxes.
"Andy has an opportunity to go out on loan as well and I know there are one or two clubs who are interested.
"Annan have been keen and I know Arbroath were too.
"I would encourage it because we need all our players to be playing, especially our younger ones."