Walcott was hooked by Pearce at halftime of their opening European Championship win over Finland and needs the winger in top form to overcome the Spanish today.
"It's impossible to lose Theo, he's such a good man," said Pearce. "I have the feeling that every player puts the squad ahead of themselves. This is about the collective getting over the finishing line, not the individual.
"If certain players are rested, through injury or tactically, that is the way it will be. I treat all 23 exactly the same. They are in or out on merit. I know, over the course of five matches, I will have to use every player."