The striker who featured as a subsitute in the games against FC Brasov and Stade Nyonnais last week, hurt his groin and his hip whilst training at Warwick University.
Unfortunately Clinton has hurt his groin and hip and had to pull out of training yesterday, so we need to get that checked out," said City boss Chris Coleman to the CT.
With the deal to sign the former Republic Of Ireland international seemingly on hold, Coleman added: "We have got to check it out and see what the problem is. "But he scored 17 goals last year, 16 of those were in the league, so it is a no brainer in terms of that.
"He had the same problem at the start of last season and after a week it was all right, but Clinton has missed most of the pre-season because he has not been with any club, so hopefully when we do get him in the door we can get him up to scratch.
"But the most important thing is with players like that you know they are going to score goals because that's what he has done wherever he has been.
"Once we find out exactly what it is, and I am sure it is nothing too drastic, hopefully we will be able to move forward and make him another new addition."
Talks with Stephen Wright are on-going. The right back is a free agent after being released by Sunderland and has impressed whilst playing for City in four pre-season matches.