"He was very excited about his first one," Mills said. "It will be his first Opening Day assignment, and he was really looking forward to it."
Mills, who didn't name his starting rotation beyond the first game, pulled Rodriguez aside Wednesday morning and told him the news.
"I'm happy because that game is a special game for me because it's my first Opening Day," said Rodriguez, who's been in the Astros' organization 13 years. "I'll try to do the best I can that day."
Rodriguez, 33, will be the Astros' third different Opening Day starting pitcher the past three seasons, following Brett Myers (2011) and Roy Oswalt, who made a club-record eight consecutive opening starts from 2003-10. Rodriguez went 11-11 with a 3.49 ERA with 166 strikeouts in 30 starts last year.
He'll be the first left-handed Opening Day starter for the Astros since Dave Roberts, who started two in a row (1973-74).
When he gets on the mound and takes the ball for Opening Day, Rodriguez said it will be just another game.
"Nothing changes," he said. "You still throw strikes. That day, I'll concentrate on where I want to put the ball. It's the same game. I don't want to put pressure on that game. It's Opening Day, but whether it's the first game or second game, nothing's different."
Rodriguez is coming off his longest outing of the spring, allowing six hits and five runs while throwing 90 pitches in six innings to beat the Tigers on Tuesday. He gave up five runs in the first two innings before settling down to retire 12 of the final 14 batters he faced.
"I told you the other day I wanted to go longer in the game, and I did it today," Rodriguez said Tuesday. "I tried to throw everything -- my breaking ball, my fastball, my changeup. I used a lot of my changeup today and threw a lot of good ones."
That was a good sign for the Astros, considering Rodriguez's struggles this spring. He's 2-2 with a 7.88 ERA in four starts heading into his final Grapefruit League start Sunday against the Tigers in Lakeland. That's the Astros' final spring game in Florida this year.
Mills wasn't too concerned about Rodriguez's spring struggles, but was glad to see him get it together on Tuesday. Last year, Rodriguez was 0-3 with an 8.04 ERA in five spring starts.
"You hate to kind of just pooh-pooh everything that happens here in spring," Mills said. "We have to put some type of importance on what happens in spring -- just not with him, but with everyone. We hope that he's at least able to get that arm in shape and build up that arm strength to get ready for the season, and he was able to do that [Tuesday]. It was sure nice to see him go those 90 pitches and get ready."
Rodriguez ranks first all-time in franchise history in strikeouts for a lefty, with 1,004, and ranks second in starts (197) and innings pitched (1,176) and third in wins (73) among southpaws. He was named the Astros' Pitcher of the Year in 2009 and '11.
The Astros signed Rodriguez as a non-drafted free agent in 1999 out of the Dominican Republic, and he made his Major League debut in 2005, the year Houston won the National League pennant. Rodriguez suffered the loss in Game 1 of the Fall Classic, allowing one run, four hits and five walks in 3 1/3 innings in relief of starter Roger Clemens.