Bale, who became the world's most expensive player when he joined Real for 100 million euros (USD 133 million) at the start of the month, played the final half an hour of Wales's 3-0 World Cup qualification defeat to Serbia on Tuesday, his first competitive action since July.
"We have looked at him in these 10 days and he is nowhere near ready for 90 minutes even if Spanish football is slower," Coleman was quoted as saying on the FIFA website on Wednesday.
"If he starts Saturday, he will not finish the game. I would be amazed if he does," he added.
"He may get 45 minutes, I don't know, but he will feel better in himself after 30 minutes here. But he has only had four training sessions in two months which is dangerous," he said.
Real have won their opening three La Liga matches under new coach Carlo Ancelotti, who replaced Jose Mourinho at the end of last season.
It remains to be seen how Bale adapts to playing alongside the likes of Cristiano Ronaldo and Karim Benzema and whether he can make up for the departure of Germany playmaker Mesut Ozil to Arsenal.
Ozil was one of Real's most creative squad members and developed a lethal partnership with Portugal forward Ronaldo.
Coleman said Real fans should give Bale time to settle. "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.
"But he gave us a lift, the fans were excited and it gave us a spur, although the game was done and dusted. We needed to be in the game when he came on. It was lost by then, but it was nice for the Welsh supporters to see him," he added.


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