But the Blue Jays had other ideas, rallying against Fields to tie the score on a Jose Bautista sacrifice fly in the eighth and winning when Nolan Reimold slammed his second homer of the game -- a solo shot off Qualls -- with one out in the ninth for a 6-5 victory on Thursday night at Minute Maid Park.
Reimold hit two of the Jays' four homers, increasing their American League-leading total to 134.
"Obviously, their four solo home runs proved to be the difference in the ballgame," manager Bo Porter said. "They definitely are a power-hitting team."
Reimold, who was claimed off waivers earlier in July, was a late addition to the lineup after Colby Rasmus showed up late and was benched.
"It definitely feels good to help get the win and contribute," Reimold said. "Not that I didn't feel part of the team before -- guys are great here and I'm really happy to be here -- but it's special to help the team actually get the win."
Buchanan did a nice job, too, working a career-high five innings and holding the Jays to three runs on five hits in his second big league start.
"You just have to roll with the punches," said Buchanan, who induced 11 groundouts and struck out three. "They told me right at 3 o'clock. We were out for stretching, and I came back in and tried to lock it in and get ready to start."
The Astros built a two-run lead after scoring three times in the third on RBIs by Robbie Grossman (2-for-3, two walks), Jason Castro and Marc Krauss. Solo homers by Reimold (in the fifth, off Buchanan) and Dioner Navarro (in the sixth, off Jose Veras) tied the score.
"It's tough," Krauss said. "Jake battled and got through five and did a good job of mixing his pitches and giving us a chance. We got ahead early and couldn't hold on and couldn't get a couple of big hits at the end of the game to really stretch the lead even more. That's frustrating, but that's baseball."
The Astros regained the lead in the sixth. Gregorio Petit missed a suicide squeeze sign but still put enough bat on the ball to put it in play and allow Matt Dominguez to score.
Fields, who had allowed two earned runs in his previous 27 2/3 innings, surrendered the lead in the eighth by allowing three hits and a run, and Reimold hit the first pitch he saw from Qualls over the left-field wall with one out in the ninth to win it.
"It's tough, but at the same time, we knew the shortage of our roster going into the game," Porter said. "That's part of it. We were able to get the ball to the guys we were able to get the ball to, but we were not able to get it done."
The Astros played most of July without starting outfielders Dexter Fowler and George Springer, two of their biggest weapons, both of whom are injured. And Enrique Hernandez, who had been a sparkplug offensively while starting in center field, was sent to Miami along with Cosart, so that was another hole.
The lineup that the Astros used on Thursday had seven players who were hitting .231 or lower, plus American League batting leader Jose Altuve and Petit, who had only six at-bats this season going into the game.
"These guys have been faced with a lot of adversity, but at the same time, when the game starts, you strap it up and you battle with the people in the battle with you," Porter said. "There's nothing you can do with anybody who's not here. That's my mind-set, and I think our guys have the same mind-set."