Altuve was out of the lineup for the second consecutive game Tuesday against the Padres because of a strained hamstring.
Altuve was prepared to bunt late in Monday's game, but the opportunity didn't arise.
"He says he feels better today," Mills said. "Those hamstring strains are tough. To make him more available than that, that would be stretching it. I want this guy to run 100 percent before he's able to get back out there and do something."
Altuve, who's third in the voting at second base in the National League for the All-Star Game, has been taking batting practice and fielding ground balls.
"I'm going to do everything to be ready, in case we need a pinch-hitter or something like that," he said.
Meanwhile, relief pitcher Wilton Lopez will throw a bullpen session on Wednesday to gauge the sprained right elbow that's kept him on the disabled list since June 11. Mills would like Lopez to make at least one Minor League rehab appearance.
"When you start facing hitters and you get up and the adrenaline starts pumping up a little bit, things are a little bit different than just throwing in the bullpen," Mills said. "That's the best way to know whether you're OK or not. If we don't send him and put him on the hill and something happens, then we screwed up and we lost some time."
Mills also said infielder Marwin Gonzalez, who's been on the disabled list since June 7 with a bruised right heel, could begin a Minor League rehab stint later this week.