Welshman Bale has struggled with injuries since his 100 million euros move from Tottenham Hotspur, missing seven of Real's 22 games with different niggles.
Ancelotti said he is still not fully recovered from a calf strain.
"He is in the squad and we need to make a decision if he plays or not," Ancelotti told a news conference.
"He is training well but we could wait a bit longer for him to be in top condition."
With Atletico Madrid's victory over Malaga on Saturday, it means Real are now eight points adrift of the top.
Angel Di Maria is likely to continue on the right wing instead of Bale while Real remain without the injured Sami Khedira and Raphael Varane.
"We are training well and I like the attitude and concentration of the players," said Ancelotti, who agreed they need to improve their defending from crosses after conceding a spate of goals from set-plays before the Christmas break.
"We have to do better at the back and concentrate more. We started off well but the situation now has been more worrying."
Ancelotti is expecting an open game against Celta.
"I know the philosophy of (Celta coach) Luis Enrique who has lots of experience and always likes to play football. It should be an exciting game and we need to be careful as they are dangerous in attack," he said.
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