Walker (elbow, shoulder) threw a 20-pitch simulated game Monday, Nick Piecoro of the Arizona Republic reports.
Walker will be evaluated Tuesday to determine how he responds to the session, which could serves as the final step before a potential return from the injured list. The right-hander has participated in a series of bullpen sessions during September in hopes of throwing at least one inning in a major-league game this season. With the Diamondbacks being eliminated from postseason contention with a loss to the Cardinals on Monday, the club may be more inclined to activate Walker and let him pitch this week.