Moehler was originally placed in the rotation a month ago when Rodriguez was sidelined with a groin injury. Instead of moving Moehler back to the 'pen when Rodriguez was ready to come off the DL, Cooper decided the right-hander should stay put.
"We just think that's the best decision at this point in time," Cooper said. "That will be our rotation -- [Shawn] Chacon, Rodriguez, [Roy] Oswalt, [Brandon] Backe, Moehler."
Moehler auditioned for a starting role during Spring Training but made the team as a long reliever, while Sampson won a spot in the rotation after the club released Woody Williams. Nearly two months into the season, Moehler and Sampson have swapped roles.
Cooper told Sampson not to view the decision as a demotion. Sampson is also likely to see much more playing time as a reliever than Moehler, who had made only five appearances out of the 'pen before joining the rotation.
Cooper is looking for stability in the sixth and seventh innings to bridge a gap between the starter and the back end of the 'pen, currently occupied by setup man Doug Brocail and closer Jose Valverde. Sampson, an efficient ground ball pitcher, may be able to fill that need. With Geoff Geary on the disabled list and Dave Borkowski struggling, Sampson could prove invaluable.
"That's the thought process at this point," Cooper said. "We've had a lot of struggles trying to get to the back. Sampson is a guy that can pitch back-to-back days. He's always been efficient with his pitches. We think he can help us."
Moehler was 2-1 with a 3.43 ERA over four starts. He pitched five innings in his first three outings and six in his last one, which translated into a win over the Phillies.
Sampson has had several solid outings this year but has also struggled. He's allowed five or more earned runs in four of 10 starts, but he also had four games in which he held opponents to two or fewer runs.
He yielded four runs over five innings in his last start against the Phillies on Sunday. That game turned into a blowout when the bullpen fell apart following Sampson's departure.
"I'm going to do the very best job I can do coming out of the bullpen," Sampson said. "[Cooper] said it may not be permanent, it may be permanent. It's a funny game and anything can happen.
"I'm not going to get down. I'm still here, I'm still in the big leagues, I'm still living the dream. I'm playing in the Major Leagues."
When Moehler joined the rotation in early May, his focus wasn't directed toward taking a teammate's starting job. That said, the 35-year-old veteran is thrilled to have received this opportunity.
"I'm not going to sit here and lie to you and say I'm not happy, because I am," he said. "I enjoy starting. I did it probably two-thirds of my career. At the same time, I told Coop I'd be happy to relieve as well. My goal is to help this team get to the playoffs."
Postseason chances were part of the reason why Moehler wanted to re-sign with the Astros last winter. He looked at the changes general manager Ed Wade made to the roster and thought the team had a legitimate chance to play in October. With four months remaining in the season, Moehler's goal is the same -- help the team reach the playoffs. If that means moving back to a relief role down the road, he'll do so.
"That's been my whole goal, to get to the playoffs," he said. "I'm in my 11th year [in the big leagues] and I haven't experienced that yet."
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.