The Real Madrid man made a cameo in his country's 3-0 defeat to Serbia on Tuesday, his first competitive appearance of the season.
Injuries and an ongoing transfer saga meant Bale featured in just 45 minutes of pre-season for Tottenham, and none of their competition fixtures once their campaign had started.
And with Bale potentially eyeing a debut for the La Liga giants against Villarreal on Saturday, his international boss is sceptical about his ability to perform at his maximum potential.
"He played 30 minutes but after 15 minutes he looked tired - but he has only had four training sessions, so you will not get what you normally get," Coleman said.
"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.
However, the attacker did pose a threat to the visitors, and Coleman hailed his positive impact on the game, and the fans.
"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."
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