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.
"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."
"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."
