The Astros will likely call up catcher Carlos Corporan as well as a pitcher or two and a few more position players.
"You want to keep supplementing the ballclub," Mills said. "You want to be able to have a guy that can pinch-run, maybe some guys to pinch-hit, maybe a couple of more left-handed and right-handed bats off the bench or in the lineup against a left-handed pitcher or a right-handed pitcher.
"Usually, the guys who come up are guys you want to get a look at and see what they can do. It's the introduction to the Major Leagues for some guys and they want to see what it's all about, see what the travel is all about, see what it's like coming to the ballpark and how much work we have to do every day."
The Astros' Triple-A affiliate in Oklahoma City is competing for a playoff spot in the Pacific Coast League, which could impact which players the Astros call up early in September.
"You don't want to take everybody from their club," Mills said. "These guys have been down there all year long and they put themselves in position to win something, and you don't want to devastate the whole club."