"I am disappointed because he should have gotten a 'W,'" Cordero said. "He pitched too good of a game to go home with a no-decision."
The loss was the eighth in a row for the Astros, who have lost 21 of their last 23 games to fall to 34-64, which is one game better than they had last year through 98 games en route to a 106-loss season.
"It was tough to give it away at the end," manager Brad Mills said.
Cordero was acquired by the Astros from the Blue Jays in a 10-player trade last week and made his Astros debut Friday in Arizona by pitching a scoreless eighth. He took over as closer from Brett Myers, who was traded after nailing down 19 of 21 save chances.
He entered the ninth inning with a 2-1 lead and got a quick out before giving up a single to Zack Cozart. Drew Stubbs followed with a home run into left field to give the Reds the lead, and Xavier Paul (3-for-5) added an RBI single after a pair of walks.
"[Harrell] threw an unbelievable game and they gave me the chance to get a save and he could get a 'W,' and I didn't. I made a mistake and Drew Stubbs hit it out and they won," Cordero said matter-of-factly. "This is very disappointing because when you have a guy pitching that way and you're not playing well as a team and that guy is pitching as well as he pitched tonight against a team that's in first place, you're disappointed."
Stubbs admitted familiarity with Cordero was beneficial.
"Playing with him for 2 1/2 years, you stand behind him and watch him pitch a lot," he said. "It helps whenever you're playing behind a guy for a few years."
Harrell has been one of the biggest bright spots this year for the struggling Astros. He worked seven innings and allowed one run while throwing a career-high 121 pitches, which also marks the most thrown by an Astros starting pitcher this year.
"I wasn't throwing a lot of strikes, and I knew when I did throw strikes I had to make quality pitches," Harrell said.
In his last three starts, Harrell is 0-1 with a 1.42 ERA. He's allowed 16 hits and struck out 16 batters in 19 innings in that span.
"He's got one of the more live fastballs that we've seen," Stubbs said. "A lot of movement, a lot of run, a lot of sink. He's pretty much able to pitch off that one thing and mix in some breaking balls and changeups. He's one of those guys that'll bust it on your hands and then back door it. Just a lot of action on his fastball."
The only runs the Astros managed came on a two-run home run by Justin Maxwell -- his 10th of the season in only 157 at-bats -- in the fourth inning to put Houston ahead, 2-0. The Astros have scored two or fewer runs in eight of 12 games since the All-Star break.
Cincinnati starter Mike Leake worked eight innings and allowed two runs and seven hits.
"They're all tough, this one even more because we felt like we were in the game and playing good behind Lucas," Astros third baseman Chris Johnson said. "He did a great job tonight. This was a tough one."