Willingham was scheduled to serve his suspension during Oakland's series opener against the Rangers on Monday night, but he elected to appeal the decision and will play until the process has been completed.
The eight-year veteran was ejected by umpire Bill Miller for arguing a called third strike and throwing down his helmet at the conclusion of the top of the eighth inning. Willingham appeared to bump Miller, but said on Monday, "I felt like there was contact, but I felt like the contact wasn't really initiated by me."
It is the first suspension in Willingham's career. He said he was unclear how long the appeal process is expected to last.
"I didn't know what to expect," Willingham said. "I've never been through any of this."
Should Willingham have to miss a game, A's manager Bob Geren has plenty of options to fill his lineup thanks to a roster stacked with outfield depth. Ryan Sweeney and Conor Jackson have each started multiple games in left field this year.
Willingham is hitting .235 after Monday's 7-2 win over the Rangers. He has a team-leading six home runs and 22 RBIs. He's also struck out 37 times, tied for third-most in the American League.