The Real Madrid man, who played just 45 minutes of pre-season for Tottenham, made a cameo in his country's 3-0 defeat to Serbia on Tuesday, in what was his first competitive appearance of the season.
With Bale eyeing a debut for the Liga giants against Villarreal on Saturday, though, his international boss is sceptical about his ability to perform at his maximum potential.
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"If he starts Saturday he will not finish the game, I would be amazed if he does. He is nowhere near ready for 90 minutes even if Spanish football is slower.
"He may get 45 minutes, I don't know, but he will feel better in himself after 30 minutes."
Bale was introduced into the match at the Cardiff City Stadium after Serbia had scored their third goal and the game had been put to rest, but the 24-year-old did pose a threat to the visitors and Coleman hailed his positive impact on the game.
"He gave us a lift," the 43-year-old added. "The fans were excited and it gave us a spur, although the game was done and dusted.
"We needed to be in the game, it was lost by then, but it was nice for the Welsh supporters to see him."