Nearly half of the Astros' 34-man roster is made up of rookies, including 10 rookie pitchers.
"There's no doubt there's some evaluating going on, but at the same time, these guys are learning to be big leaguers and what it's like to pitch against Major League teams," Mills said. "You've got to be able to make pitches, repeat your delivery, repeat that release point. That's what they need to do.
"We've seen some guys in the last two weeks who have struggled in doing that, and they've run into problems. At the Major League level, you're not going to get away with balls coming up in the zone or throwing through your movement on certain pitches. Even for the non-pitchers, these guys have to learn how to make adjustments and learn what it takes to keep their focus where it needs to be."
That being said, Mills said the composition of his bullpen for next year will have more to do with performances during Spring Training than what happens in the final two weeks of the season.
"We've always heard you can't evaluate too much in Spring Training or September, but we can get an idea and we can kind of have our wish list," Mills said. "We want guys to earn spots in Spring Training as we move forward."