Real Madrid's new £86m signing was an unused substitute in Friday's 2-1 loss against Macedonia, prompting anger from some supporters.
Manager Chris Coleman insisted the star winger was injured when questioned over his selection in Skopje but it was only on Monday that he revealed Bale had picked up a groin problem and was to undergo a scan.
Following that check, the former Tottenham player was fit enough to take part in training ahead of the Cardiff clash but, while he will be assessed in the run-up to kick-off, he will make the bench at best.
Speaking earlier to Sky Sports News, Coleman said: "With him not playing any games he's not going to start on Tuesday and he never was.
"When you've had three to four training sessions in six to eight weeks, as good as he is, you can't be risking players."
Coleman had suggested the scan was to assure Bale he would not be putting himself at risk ahead of a potential Real Madrid debut against Villarreal this weekend.
"We can't alter the fact that the pitch wasn't great and he just feels something in his groin - a strain," Coleman added of Bale. "He came to see me on Friday morning and said something didn't feel right.
"The scan is precautionary but he needs to know and be sure that there's nothing there. We don't think there is. If it's good news and he can play some part, of course he will. But ultimately the player says if he can or he can't."
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