But he believes that Leigh Griffiths has boosted his chances by settling in so quickly at Celtic following his January move from Wolves.
Griffiths was called up to the Scotland squad for the friendly against Poland on Wednesday following the withdrawal of Robert Snodgrass and Grant Hanley, during a weekend in which he hit his first hat trick for Celtic.
McGhee, though, warned that the striker would need to maintain his form up to the start of the qualifiers, which begin against Germany on September 7. "There's a fairly limited number of players we feel are going to be good enough to help qualify us," McGhee said.
"Leigh Griffiths is one of them and we think that Leigh being at Celtic, and the new experience he will have in the Champions League, can only make him a better player and more prepared for the national team.
"He has obviously risen to it and hit the ground running. We would expect him to get a lot of goals between now and the end of the season, and then it will be about the start to the season.
"Maybe [Craig] Mackail-Smith might be fit by then, and we have got Ross McCormack, Steven Fletcher, Leigh Griffiths, Jordan Rhodes. It's not about who is playing well now, it's going to be about August and September."