Walker officially announced his retirement Tuesday.
Walker elected free agency Monday after refusing an outright assignment to the Phillies' alternate training site.
Walker was designated for assignment by the Phillies on Friday.
Walker went 3-for-3 with two RBI Wednesday as he played part in in a 3-0 win over the Nationals.
Walker will start at first base and will bat eighth Wednesday against the Nationals, Scott Lauber of The Philadelphia Inquirer reports.
Walker will start at second base and bat seventh in the first game of Sunday's doubleheader with the Braves.
Walker will be on the Phillies' Opening Day roster, Matt Gelb of The Athletic reports.
Walker will be part of the Phillies' 60-man roster.
Walker hit a passable .250/.300/.393 in 11 Grapefruit League games this spring.
Walker signed with the Phillies as a non-roster invitee Wednesday, source reports.