Wallace was drafted as a third baseman before being moved to first base, which is where he played for the Astros last season. He took up third again this spring but has limited experience at the position at the Major League level.
"Wally played a little bit there this spring and looked pretty good, so we just kind of want to give him some time down there and see how he does, see if that's an option for us," Mills said.
Mills also didn't rule out Wallace seeing some time at shortstop, where he appeared in 10 games at Triple-A Oklahoma City this season before being called up. He has never appeared at shortstop in the Majors.
"We want to answer as many questions as we can, that being one of them," Mills said.