"That became a big roadblock," he said.
The Astros limp into the second half of the season in the now-familiar position of holding baseball's worst record. Mills' biggest task is to help his young team find a way to win on the road, though they will have to do it without the leadership of veteran Carlos Lee, who was traded to Miami last week.
And Houston will start the second half in daunting fashion: a 10-game West Coast swing to face the Giants, Padres and Diamondbacks.
The difference in the Astros at home, where they are 24-21, and on the road is so stark that it sometimes doesn't even seem like the same team.
Consider these numbers: The Astros have hit a league-low 27 homers away from Houston but are 11th with 49 long balls at home. Their 149 runs on the road are tied for worst, while the 195 they've piled up at home rank ninth.
And it's not just the offense that has been inconsistent. Houston's 5.66 road ERA is the highest in the majors. At home, the staff has posted a 3.47 ERA -- the 10th lowest. Teams are hitting a baseball-high .297 against the Astros on the road, but managing just .243 at Minute Maid Park.
Mills has talked with his players and staff endlessly to try and fix this situation. In the end, he decided there's one thing that should help turn things around.
"Just consistent play," he said. "We see us do it one time and we want to be able to continue it. I think (inconsistency) comes from having a lot of young players. So if we can learn to get it consistent to where we can do it here at home and then take it on the road then we'll be much better."
The players are well aware of the issue. Chris Johnson, who started the season at third base but took over at first when Lee left, believes more attention to detail could be a cure for the road woes.
"We need to play good defense, we need to run the bases well and get good, timely hitting," he said. "We're the kind of ballclub that we need to do all those little things to win, and if we don't, we're going to lose."
Houston is also grasping for leaders after the departure of Lee. Though the 36-year-old Lee didn't talk much, Mills believes the Astros will miss simply getting to watch him hit every day and learn from his approach.
"There's no doubt that it does change the complexion of the ballclub," Mills said.
But Mills doesn't need anyone to try too hard to fill Lee's leadership role, he just wants each player to be individually accountable.
"They don't have to put so much emphasis on being a leader on the ballclub," he said. "But if they lead themselves, and just do the things that they need to do ... go out and play the game hard and when you do that, that's being a leader."
Houston has struggled to find an identity in the last few years as star after star has been traded away. First to go were Roy Oswalt and Lance Berkman, and last summer Hunter Pence and Michael Bourn followed. Lee's departure marked the end of an era, and left Houston with no position players who were on the team full-time before 2010.
Houston might not be done shedding its older players with an eye toward building for the future. Veteran left-hander Wandy Rodriguez and closer Brett Myers could be attractive to teams looking for a push into the postseason.
No move would surprise starter Bud Norris, who has been around through all of the aforementioned trades.
"I came up in `09 and I think we were the second-oldest team in baseball, and now I think we're the second-youngest team," he said. "So I've seen a lot of transition in this clubhouse, which has been interesting. I know we needed a fresh start and we're excited for the future."
A bright spot in Houston's tough first half has been the emergence of second baseman Jose Altuve, who at 5-foot-5 is the shortest player in the majors. Altuve earned his first trip to the All-Star game by leading all National League second basemen in batting (.303), hits (96) and steals (15).
He has teamed with shortstop Jed Lowrie to make Houston's middle infield one of the strongest in baseball. Lowrie is in his first season with the Astros after an offseason trade with Boston. His 14 home runs are the most by a shortstop in the first half of the season in franchise history and second-most by shortstop in the NL this season.
The bullpen also did a good job through most of the first half before getting a bit off track after key middle reliever Wilton Lopez was injured in early June. Lopez, who has a 2.51 ERA with a 3-0 record in 32 appearances, is set to return after the break and Mills expects that to be a boost to his team.
The Astros are also hoping centerfielder Jordan Schafer, who was acquired last summer in the trade for Bourn, rebounds. Schafer hit .276 in April before injuries and illnesses derailed his development and left him with a .176 average in June and hitting .235 overall.
"I've been kind of lost for the last month," said Schafer, who was also pulled from a game in July for fielding mistakes. "It hasn't been fun. But hopefully I can take the break and get away and clear my mind out a little bit and come back and get back to how I was the first month of the year."
Mills is confident that the speedy Schafer, who is tied for fourth in the NL with 20 stolen bases, can bounce back and finish the season strong.
"Here's a guy that has all kinds of talent and everyone sees it, and we want him to be able to utilize that talent to the best of his capability to help him and help the ballclub," Mills said. "Because when he does use those skills, he's an asset to any ballclub. We want to get all these young kids to a situation to see how important it is they work together and see the things we can accomplish if we do work together."