The mascot for the Boston Red Sox is a green monster named "Wally". And since he's a monster, I suspect he can go to Monsters Inc. and back to Fenway Park through these doors. Since there's not a game going on, Wally invites Sulley and Mike over to have the park for themselves.

Art by me
Wally the Green Monster © Boston Red Sox
James P. Sullivan and Mike Wazowski © Disney & Pixar