Bale's absence, if confirmed, would allow Ancelotti to put off having to make a tough choice between James Rodriguez or Isco in midfield alongside Toni Kroos and Luka Modric.
James played for the first time after a two-month injury layoff in Sunday's 9-1 drubbing of Granada and Isco is back from a one-match suspension, meaning Ancelotti could use both playmakers in his starting lineup, with Cristiano Ronaldo and Karim Benzema up front.
"We have had a problem with Bale, who has a knock on the left foot," Ancelotti told a news conference.
"Both (James and Isco) could play because they have played together many times," added the Italian.
"It is not a debate about Isco or James but a debate about a squad in very good shape.
"It is very hard to choose between players of such quality because they are two complete players who can play in various situations on the pitch.
"I have the good fortune to have both of them, what a great problem to have."
Real could find themselves seven points adrift of Barca when they take to the pitch at Rayo's stadium in the Madrid suburbs.
Barca, who edged a 1-0 win at Celta Vigo on Sunday, play at home to Almeria earlier on Wednesday.
Barca have 71 points with nine games left, with Real on 67 and champions Atletico Madrid, who host Real Sociedad later on Tuesday, five points behind their neighbours in third.
Modest Rayo have out-performed again this term and are a heady ninth after Friday's 2-1 win at Eibar.
"They are on their best form of the season," Ancelotti said.
"Their position in the standings speaks for itself. We are well aware of the difficulties the game might present."
(Writing by Iain Rogers, editing by Ed Osmond)
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