The winger played the final half-hour of Wales's 3-0 World Cup qualification defeat at home by Serbia on Tuesday - his first competitive action since July - and trained with his new team-mates for the first time on Wednesday.
"I think everyone knows his pre-season was not normal but his physical condition is not bad," Ancelotti told a news conference in Madrid on Friday.
"I don't know if he will start but he will play some of the match. He is very excited and very motivated."
Speaking at the launch of Real's third kit on Thursday, the 24-year-old Bale insisted he would be ready to answer Ancelotti's call on Saturday night.
"I feel good," he said. "I have been training in the last week with Wales. I have had two good sessions today and I got half-an-hour with Wales on Tuesday.
"Obviously I am a bit behind everyone else physically, but I am hopeful I can catch up as soon as possible.
"I think you will have to wait until the match to see whether I am picked or not. Hopefully I can get my fitness up straight away and hopefully get a place in the team."
Wales coach Chris Coleman expressed his view after the Serbia defeat that Bale is still not in a position to play a full 90 minutes.
Coleman had said: "If he starts Saturday, he will not finish the game. I would be amazed if he does.
"We have looked at him in these 10 days and he is nowhere near ready for 90 minutes even if Spanish football is slower.
"He may start, but he won't finish the game. He may get 45 minutes, I don't know, but he will feel better in himself after 30 minutes here.
"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."
After the Villarreal fixture, Real face Galatasaray in the Champions League on Tuesday before hosting city rivals Getafe at the Bernabeu the following Sunday.
Villarreal v Real Madrid is live on Sky Sports 2 this Saturday, kick-off 9pm (UK time)