Happ, who came to Houston in the trade that sent Roy Oswalt to Philadelphia in July 2010, is 4-14 this season and has struggled with a 6.26 ERA in 22 starts. He has allowed five or more runs in eight straight starts.
"I'm disappointed," Happ said. "You never expect to be sent down. Hopefully I can go down and clear my head a little bit and get this thing figured out sooner than later. It's way too late to start feeling sorry for yourself. I'm going to go down there and do what I've been trying to do. I've been working as hard as I can."
Houston also placed reliever Enerio Del Rosario on the 15-day disabled list with a strained right shoulder and called up left-handed reliever Wesley Wright and righty reliever Jeff Fulchino from Oklahoma City.
Happ has lost 10 of his last 11 decisions, with his last win being July 19 against Washington. He threw four innings Friday against the Brewers in an 8-1 loss, allowing six runs and seven hits.
Houston manager Brad Mills said Happ needs to regain confidence in Oklahoma City.
"He needs to get where he needs to be to help us," Mills said. "Everybody knows and he knows his stuff is good enough to pitch here and pitch well here. It's just that he needs to get that confidence to be able to throw it and let the ball work for him."
Happ's turn in the rotation is scheduled to come up again on Wednesday, and both Mills and Astros general manager Ed Wade said the team has options for who might start, including right-hander Aneury Rodriguez, who is 0-4 with a 4.78 ERA in 30 games, including eight starts.
Happ was 5-4 with a 3.75 ERA in 13 starts last season for Houston.
Wade said the move was hard to make.
"This was something that we contemplated a couple different times about what is the best way to get J.A. back on track," he said. "It was not an easy decision. At the end of the day, we thought maybe this was the best action to get him out of this environment and get him to a point where he can relax and make his pitches."