Manager Cecil Cooper said Tuesday that Brian Moehler, who's made four solid starts since Rodriguez was sidelined by a left groin injury on April 20, will be the fifth starter. That pushes Sampson, 3-3 with a 6.04 ERA to the bullpen.
"He says at this point it's maybe not permanent," Sampson said. "It may be permanent. This game is a funny game and anything can happen.
"I'm not going to get down. I'm still here in the big leagues, still living a dream."
The 36-year-old Moehler is 2-1 with a 4.40 ERA after signing a minor league contract in the offseason. Since moving into the rotation, he's 2-1 with a 3.43 ERA, the first three starts lasting five innings and the last one six innings as he increases arm strength.
"He's thrown strikes, he's done some things we've been preaching from day one in spring training," Cooper said. "When you've got your fourth or fifth guy, all you want them to do is keep you in the ballgame, and that's what he's done."
Moehler made 42 relief appearances last year for the Astros, but made 46 starts for the Marlins in 2005-06 and reached double digits in victories four straight years as a starter for the Tigers from 1997-2000. His next start is scheduled for Saturday at Milwaukee.
"I'm not going to sit here and lie to you and say I'm not happy, because I am," Moehler said. "At the same time, I told Coop I'd be happy to relieve as well.
"My goal is to help get this team in the playoffs. This is my 11th year and I haven't experienced that yet."
Cooper said he told Sampson not to think of the move as a demotion. He also said the Astros are in need of stronger middle relief. Sampson was 7-8 with a 4.59 ERA last year, his first extended time in the majors.
"We've had a lot of struggles trying to get to the back [of the game], and this we think gives us the best chance," Cooper said.
Rodriguez threw 78 pitches over six innings in his only rehab start, allowing one run and four hits for Double-A Corpus Christi. He's 1-0 with a 2.31 ERA in four starts, heading into Wednesday's return against the St. Louis Cardinals' Adam Wainwright.
Right-hander Dave Borkowski was designated for assignment, two days after allowing six runs in one inning at Philadelphia, to make room for Rodriguez. Borkowski, 31, was 0-2 with a 6.86 ERA in 18 relief appearances and allowed five homers in 21 innings.
"He pitched some big innings for us last year and the year before," Cooper said. "This year he just was not consistent enough. We put him in some key spots and he didn't deliver."
Another rehabbing pitcher, right-hander Geoff Geary, was scheduled to throw a bullpen session on Tuesday and could rejoin the bullpen next week. Cooper said he didn't think Geary, sidelined since May 16 with a strained right groin, would need a rehab assignment since he's not been out long.
Geary is 1-1 with a 1.47 ERA in 19 games, totaling 18 1/3 innings.