"We're keeping our options open," assistant general manager David Forst said before Friday's game against the Giants. "Bob Geren talks about how versatility is going to be important on this club. We're trying to have as many options for him as possible."
Walker has mostly played second base since coming up with the Minnesota Twins in 1996, but he has played 49 games at first base in his career.
On Friday, Nick Swisher played first and Travis Buck played right field. Walker would likely be an emergency scenario, if something were to happen to Erubiel Durazo or Swisher. Designated hitter Mike Piazza is also another option at first.
When Forst was asked whether he thought Walker could be in an A's uniform during the regular season, he said, "Anything is possible."
Walker was with the Padres this spring before being released on Monday. He batted .225 with one RBI in 14 games this spring. The 33-year-old split his 2006 season between the Chicago Cubs and San Diego and batted a combined .278 with nine home runs and 53 RBIs in 138 games.
The left-handed hitter from Louisiana State University is a .289 career hitter and could be playing for his seventh big-league team.
Johnson suffered torn cartilage in his left hip in a Cactus League game collision with Colorado's Yorvit Torrealba on Monday.
Ryan Quinn is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.