McManus claims the Parkhead club found a problem with Turnbull's knee which will require an operation and 10-12 weeks on the sidelines.
This was the latest development in a long twist of events ever since Celtic declared their interest in the Motherwell playmaker.
Despite this latest set-back, McManus insists Turnbull will become a Celtic player this summer.
He wrote on Twitter: "David Turnbull has a small problem with his knee that requires an op (tidy up). Be out for 10-12 weeks. That’s it. Some of the s***e I have heard about him on here is poor & embarrassing. Hurtful rumours about the kid. The deal will 100% still go through."