Santander captain Mario Fernandez announced on Monday they would refuse to take the pitch against Real Sociedad if the current board remain in place.
The players cited four months of unpaid wages as the reason for their decision, and called for the club's president Angel Lavin to step down.
Santander go into the second leg 3-1 behind but the deciding tie may not get started as the players union (AFE) confirmed the strike would still go ahead unless their demands were met.
"If at 2100 (CET) the Racing board is the same as now, they (the players) will not play," AFE president Luis Rubiales confirmed having met the squad.
"The squad decided something last Monday which they made public, and they continue thinking exactly the same, that if the current board have not resigned by the time of the game they are not going to play.
"They have the complete backing of the AFE, as always."
The players were warned that not competing could have implications regarding the competition, but Rubiales expects them to remain defiant.
FIFPro, who represent players on a global scale, also backed the players' cause.
"FIFPro supports the fundamental rights of the players to have their contracts respected. That they should expect to receive their salary each month is non-negotiable," they said in a statement.
"AFE is digging deep to provide the players of Racing Santander with temporary financial support. It will provide one month's salary from its emergency fund to help all concerned buy a little extra time while they continue to fight for their outstanding wages."
