Walcott is currently out until mid-February with a shoulder injury.
Pearce said: "He's one of our best players. He's proved that because he's playing in the seniors now. It's vital he's there.
"I didn't see Holland, the last winners of the Under-21 tournament, leave Ryan Babel out even though he had senior caps.
"As an Englishman it's important I take the strongest squad to try and win the tournament.
"So when you say 'Would I even ask for Theo Walcott?' it's not about 'even Theo' - it's 'especially Theo'."