Real's trip to Villarreal is expected to mark Bale's first appearance in the club's famous white shirt since completing his world-record move from Tottenham at the end of the transfer window.
But the 24-year-old did not feature in Wales' 2-1 defeat in Macedonia on Friday after picking up a groin strain, and underwent a scan on Monday.
He was later able to take a full part in Wales' training session ahead of Tuesday's World Cup qualifier against Serbia, but a final decision over his fitness will only be made on the night.
If Bale plays no part in Cardiff, his previous match action will have been back in mid-July.
And Coleman does not believe Bale can start at El Madrigal on Saturday given his lack of training and game time.
Coleman said: "To have four sessions and then play a game like we have got is tough. La Liga is a different tempo to the Premier League, but I'd be surprised if he starts.
"I understand they've got a problem with Cristiano Ronaldo, but it's a big, big ask to do four training sessions [and then play].
"He is a super athlete, he's not got an ounce of fat on him, but even for him, if you see him in the sessions with us - and we're not full pelt - he's not quite there.
"He's only had four sessions and one of them was by himself with our physio last month, before the Republic of Ireland game. But it will be up to him. If he says he can play a part, he'll be on the pitch no doubt."
Coleman added: "Physically, it's a big gamble. Especially if he plays some part for us and then he has to travel back."
But Coleman will not hesitate to use Bale himself, provided he is fit enough to face Serbia.
He said: "If Gareth says to me he can play a part then he is coming on to the pitch. There is no way that he is not coming on to the pitch.
"I would have used him in the previous game, if he had said to me he was ready before the game.
"If he had said to me he could play 20 minutes, then I would have put him on for 30, I would have pushed him a little.
"But I can't do that if he says to me he isn't ready.
"If there is any chance he can play for a second - it only takes a player a second to change your life - he will be on the pitch.
"I can't start him, he's not ready. He's had four training sessions in eight weeks.
"I don't care how good anyone is. If you want to take a risk starting a player you're asking for trouble.
"And he's not our player, don't forget. He belongs to Real Madrid. We are mindful of that.
"I have to take Gareth's word. He always turns up for Wales - if he didn't want to be here then he wouldn't be here. God knows he's got excuses not to be."