The world's most expensive footballer has been nursing a leg problem and missed the cup win over Espanyol last week as well as Sunday's league draw against Athletic Bilbao.
Bale has completed 90 minutes just nine times in 30 competitive games this season, following his arrival from Tottenham, and a strong performance from whistle to whistle by the Welshman against Atletico will be sought after by head coach Carlo Ancelotti.
Atletico are the surprise Primera Division leaders after winning 18 of their 22 matches to date, a record which has taken them three points clear of Barcelona and Real.
The cup tussle over two legs offers Real an opportunity to strike a psychological blow, and with Atletico boss Diego Simeone playing down his team's league title prospects, the battle in the minds of the managers is already being fought out.
Speaking on Tuesday, Ancelotti frowned on his opposite number's suggestion.
Ancelotti said: "Simeone doesn't think that his team are contenders. He's saying that to ease the pressure.
"He's done a fantastic job. Atletico are going very well. They started as underdog and now they're top of the league, in the Copa semi-finals and the last 16 of the Champions League."
Cristiano Ronaldo is expected to line up at the Bernabeu, but the FIFA Ballon d'Or winner faces a possible three-game ban after being sent off against Athletic at the weekend.
David Villa's hamstring injury is expected to keep the Atletico striker sidelined, while midfielder Tiago Mendes is a doubtful starter due to a muscle strain.
Barcelona conceded the league leadership after their surprising home defeat to Valencia on Saturday, and they tackle Real Sociedad in the second semi-final, beginning an hour after the Madrid derby commences.
Head coach Tata Martino stressed being in charge of Barcelona always brings a challenge, whether the team head the league or not.
"From the day I came to Barcelona until today, every day has been equal, the pressure is always the same," he said in a press conference, according to Marca.com.
"The next match gives you a chance for vindication. And more so in a cup semi-final. It's very important that we take a lead to San Sebastian."
Barcelona remain without the injured Neymar but have named a 22-man squad for the match, Martino keeping every option open.
Sociedad boss Jagoba Arrasate had a simple message for his underdogs ahead of the first leg in the Nou Camp.
"This generation wants to play in a final," he said. "Words won't make that happen, we have to do it on the pitch."