Ortiz will start on Saturday against the Texas Rangers in place of Felipe Paulino, who is listed as day-to-day after straining his right groin on Sunday.
Paulino was taken out after only 34 pitches. He was hitting 98 mph on the radar gun and had given up three hits and one run while striking out four in 1 2/3 innings.
"[Paulino] was sore today, more so than what he was after Sunday," manager Cecil Cooper said. "We'll just have to see how things go the next couple of days."
As a starter this season, Ortiz is 2-1 with 5.82 ERA and hasn't allowed more than three runs in any start. His last start came on May 7 against the Cubs.
While Paulino is definitely out for his next start, the Astros are close to getting a few other players back off the disabled list. Second baseman Kazuo Matsui and closer Jose Valverde will start rehab assignments at Double-A Corpus Christi on Wednesday.
Matsui took part in a variety of drills Tuesday at Minute Maid Park and starting Wednesday will play in four games for Corpus Christi before rejoining the team in Arlington for the start of the Astros' series with the Rangers.
"Kaz Matsui looked very good today and he's totally on board with this," Cooper said. "He looked very good. Once he got warm, he ran very well. We just need to see him get back and back in the swing of things."
Cooper said Matsui would play five innings the first game, seven the second and then nine innings the last two games. Valverde is expected to throw Wednesday and Thursday before rejoining the Astros before they leave for Arizona.
Reliever Geoff Geary also threw a simulated game Tuesday and feels he is ready to return whenever the front office and coaching staff give him the opportunity.
"I feel I'm throwing the ball more effectively than I have before," Geary said after he threw two 20-pitch innings against teammates. "It's out of my hands from here, but I'm ready without a doubt. All I can do is wait until they make a decision."
Cooper said he thought it was the best Geary has thrown in his rehab. With Matsui, Valverde and Geary ready to be activated within the next week, the Astros will have to make decisions on how to free up space to make room for their key players coming back.
"We'll have to make some tough decisions when that time comes," Cooper said.
Jason Grodsky is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.