HOUSTON -- Astros first baseman Lance Berkman said on Saturday that his sore left knee has not improved since suffering a setback on Tuesday, casting doubt on whether he will be able to return next weekend when he is eligible to come off the 15-day disabled list.

"It's pretty much the same," Berkman said from the clubhouse during Saturday's game. "I hope to make an improvement, but it hasn't yet.

"It's hard to say [a return date]. I can bend it, but it's just a matter of bearing weight on it. And once you get to that point, you're still three to four days away from where you can do much."

The Astros placed the five-time All-Star on the disabled list retroactive to March 26 after Berkman's left knee swelled during an attempted workout on Tuesday. He will be eligible to come off the disabled list on April 10 against Philadelphia -- Houston's fifth regular-season game.

Berkman had arthroscopic surgery on the knee on March 13 to clean out loose cartilage, and the Astros said the timetable for his return would be two to four weeks. April 10 would be four weeks from the procedure.

"It's a slow process," Berkman said. "It's frustrating. Everyone wants an answer and I don't have one.

"I've had it drained four times, and I think we're going to do it a fifth time to try and get the swelling out of there. That's the biggest battle right now."

Veteran infielder Geoff Blum will start in Berkman's place at first base in the Astros' home opener on Monday against the San Francisco Giants.

Astros finalize 25-man roster

HOUSTON -- While the Astros had previously announced their intentions for the final 25-man roster, general manager Ed Wade made it official Saturday by purchasing the contract of outfielder Cory Sullivan and re-assigning outfielder Jason Bourgeois to Triple-A Round Rock.

Both were in Spring Training as non-roster players.

Sullivan spent last season as a reserve with the New York Mets, batting .250 with a .338 on-base percentage in 64 games. He hit .286 in 72 games with the 2007 National League champion Colorado Rockies, an accomplishment that caught the eye of Astros manager Brad Mills.

"He's a professional player," Mills said. "He's been to a World Series and been on teams that have been successful.

"As a player, he's a pretty good left-handed bat that makes pretty good contact. Defensively, he plays outstanding at all three [outfield] positions. If you can add a guy like that to the mix, it's good."

Sullivan and Jason Michaels will back up outfield starters Carlos Lee, Michael Bourn and Hunter Pence.

Mills also noted that Felipe Paulino felt "great" and was on track to begin the regular season as the No. 5 starter in the Astros' rotation.

Paulino was originally scheduled to start Saturday's game, but was scratched after suffering lower back spasms on Wednesday. Mills said Paulino asked to pitch on Saturday, but the Astros opted to give him additional rest heading into the season.

Lance Berkman and right-handed pitchers Alberto Arias and Yorman Bazardo will start the season on the 15-day disabled list for Houston.