Bale was jeered on Saturday when he came on as a 74th minute substitute during Madrid's 2-0 LaLiga win over Getafe, in his first appearance at the Bernabeu since February 2020.
The forward has played just five times for Madrid in the league this season. Fans were angry when he was declared unfit for the team's Clasico clash with Barcelona last month, only to start for Wales -- scoring twice in a World Cup playoff with Austria -- four days later.
"When you whistle one player, you whistle all of us," midfielder Casemiro told a news conference on Monday ahead of Madrid's Champions League quarterfinal second leg with Chelsea. "Football is about opinions, everyone has theirs, but I don't agree when a player on my team is whistled. We're all on the same side, we have to support him."
Bale's contract at Madrid will expire this summer, bringing an end to a nine-year stay in Spain that has seen him win four Champions League titles -- scoring in the 2014 and 2018 finals -- but also attract criticism for his injury record and perceived prioritising of his national team.
The 32-year-old has also faced criticism in the Spanish media and slammed their coverage as "disgusting" after he was labelled a "parasite" by newspaper Marca after Madrid's Clasico defeat.
"I didn't like the whistles the other day for Bale," Casemiro added. "He's a historic player for this club. He's scored a lot of important goals for this club. When you whistle a player like that, you're whistling the history of this club. I didn't like it. I think the fans have to support us."
Madrid will defend a 3-1 lead at the Bernabeu on Tuesday after Karim Benzema scored a hat trick in the first leg at Stamford Bridge last week.
The France international has scored 11 Champions League goals this season -- his best ever tally -- to add to his 24 goals in LaLiga.
"Right now, Karim represents what a modern centre forward must be," Madrid manager Carlo Ancelotti said on Monday. "In the past, a centre forward was someone who waited in the box to shoot.
"Karim is what modern football demands from a forward today, it's not just about shooting and scoring, but also helping the team, doing defensive work. In my opinion he's the perfect representation of what a centre forward must be today."