Bale missed Tuesday's cup win over Espanyol due to a leg problem and he has been training apart from this team-mates for most of the week.
Ancelotti had initially hoped the world's most expensive player would be availabe for the testing trip to fourth-placed Athletic, but confirmed on Saturday that Bale would not be involved.
However, the Madrid boss is optimistic the Wales international will be back in time for the semi-final first leg against Atletico at the Bernabeu on Wednesday, which is a repeat of last year's final that the Rojiblancos won 2-1.
"He's not available for tomorrow because he didn't train throughout the week with the squad," said Ancelotti at his pre-match press conference ahead of the trip to San Mames.
"He's done a good job today, he'll train tomorrow and I think he'll be available for Wednesday."
Bale has been hampered by niggling injuries since his record-breaking 100million euro move to Madrid from Tottenham last summer, including thigh and calf problems.
That has forced him to sit out a number of matches for the Spanish giants, but Ancelotti insists he is not concerned about the 24-year-old's fitness issues.
"He doesn't need another pre-season, the problem that he had was very small, he only needs to train and gain confidence," he added.
"I'm not worried about his problems. He had a knock and was out for 20 days. After that he played well and now he's had a small problem, but not an injury. The training that he's done shows that he's fine."
Despite his absences, Bale has notched 10 goals in all competitions already this season and has also created a number of assists for his team-mates to help establish Madrid as the top-scoring team in La Liga with 60 goals from 21 games.