The 19-year-old caught Real's eye in Racing's 1-0 defeat at the Bernabeu in November, in which he had an effort wrongly ruled out for offside, and agreed a six-year contract in February that will take him to the Spanish capital in July.
Some Racing fans were annoyed when he subsequently donned a Real shirt for a newspaper interview but club captain and former Real player Pedro Munitis is sure Canales will give his all in Santander on Sunday.
"I am certain he'll put 200 percent into the clash and he'll bust a gut for Racing as he does in all the matches he plays," Munitis told a news conference.
"Nobody should be upset if he scores and celebrates the goal because Racing is still his team."
Real host Barca for the potentially decisive second "Clasico" of the campaign on April 10, after which they will be separated by head-to-head record instead of goal difference if they are still tied on points.
Athletic are one of only six teams to avoid defeat to Pep Guardiola's side this season thanks to November's 1-1 draw in Bilbao and visit the Catalan capital ahead of Barca's Champions League quarter-final second-leg against Arsenal on Tuesday.
Defender Daniel Alves said after last weekend's 1-0 win in Mallorca he had deliberately got himself booked so he would be suspended for the Athletic match and not the Real game and Guardiola is also without injured midfielder Andres Iniesta.
"Barcelona have two very significant absentees in Alves and Iniesta but you can't compare their bench with other teams," Athletic striker Gaizka Toquero told a news conference.
The Basque club are fighting for one of four spots in next season's Champions League with clubs including third-placed Valencia, Real Mallorca (fourth), Sevilla (fifth) and Deportivo Coruna (seventh).
