HOUSTON -- After having surgery on his right knee Friday, All-Star relief pitcher Ryan Pressly said Saturday his goal is to return to action before the end of the regular season. Pressly wouldn¡¯t say exactly what the surgery addressed, except that ¡°they went in there and cleaned some things up.¡±

Astros president of baseball operations and general manager Jeff Luhnow said Thursday that Pressly would be out four to six weeks. The regular season ends in five weeks, and Pressly said there¡¯s a plan to come back before the regular season concludes.

¡°Or at least that¡¯s the way I¡¯m going to push myself,¡± he said.

Pressly took a hard grounder off his knee in July and was eventually placed on the injured list from July 28-Aug. 9 with right knee soreness. He had appeared in six games since returning and was scoreless in five of those outings, though he gave up a grand slam on Aug. 14 at Chicago. In his last outing Tuesday against the Tigers, Pressly said he felt awkward.

¡°I went to go look at film and come to find out I was striding a half a foot shorter than what I normally do,¡± Pressly said. ¡°I told [head athletic trainer] Jeremiah [Randall] that something¡¯s not right. We went and had it looked at, and kind of confirmed our suspicion of why it wasn¡¯t cooperating with us. We thought we had turned a corner by now, but still hadn¡¯t. We went in there and cleaned everything up, and now we¡¯re ready to go.¡±

Pressly was one of the best relievers in baseball in the first half of the season and made his first All-Star team. He has a 2.50 ERA, 0.93 WHIP and had struck out 65 while walking only 11. He leads the Majors in OPS against lefties at .354 (minimum 80 batters faced). Getting him back healthy will be key for the Astros when the playoffs start.

¡°It¡¯s always frustrating when you miss time, rather it being in Spring Training or offseason, or even now,¡± Pressly said. ¡°It¡¯s frustrating for me because I want to be out there with my guys. I want to be out there with my bullpen and my teammates. I want to be out there contributing to this team. That¡¯s the way I look at it. Stuff like this happens. It¡¯s a baseball injury and you just have to figure out a way to get back as fast as you can.¡±

Valdez to start Sunday

The Astros will turn to left-hander Framber Valdez to start Sunday¡¯s series finale against the Angels at Minute Maid Park, manager AJ Hinch said. Valdez, who will be called up from Triple-A Round Rock, will be filling the rotation spot that opened when Aaron Sanchez went on the injured list with discomfort in his pectoral muscle earlier this week. Valdez will also start Aug. 31 at Toronto.

Valdez last started for Round Rock Monday in Memphis, giving up four earned runs and four hits in 7 1/3 innings, while striking out eight batters. He struck out 13 in 6 2/3 innings on Aug. 12 at Albuquerque. In 20 games with the Astros this year, including five starts, he has a 5.58 ERA in 50 innings. He last pitched for the Astros on July 15 in Anaheim, and gave up seven runs (four earned) and six hits in four innings.

¡°He¡¯s been pretty good,¡± Hinch said. ¡°He¡¯s gotten nicked for a few runs late in his outings, but he¡¯s really done a good job of getting into games and pitching rather deep into the games at Triple-A. The way the offense has exploded in Triple-A, it¡¯s been nice to see him have a little bit of success, and be able to control some damage.

¡°He¡¯s had some games where he¡¯s punched out double digits, had some games where he¡¯s kept the ball on the ground. He¡¯s given up a few runs late in his outings, but he¡¯s earned the opportunity to come back and help us.¡±

Valdez began the season in the Astros¡¯ bullpen and was sent to Triple-A on April 11, returning 10 days later. This will be his fourth stint on the big league roster this year.

Astros discuss callups

With rosters able to be expanded in a week, Hinch and Luhnow met recently to discuss which players the club might summon from Triple-A. Teams can add any players on the 40-man roster beginning Sept. 1.

Hinch said any September callups would be players who have already had big league time, a list that includes pitchers Rogelio Armenteros, Dean Deetz, Cionel Perez and Jose Urquidy, as well as catcher Garrett Stubbs and outfielder Kyle Tucker. The Astros also will get infielder Aledmys Diaz back from the injured list on Tuesday, while hard-throwing righty Josh James is rehabbing with the Express and should return in September, as well.

¡°There¡¯s guys like that, and there¡¯s going to be some guys that return from having helped us before,¡± Hinch said. ¡°I don¡¯t see us adding anyone who¡¯s not been here before, but I think it will be in a couple of different waves.¡±

Round Rock entered Saturday in first place in the Pacific Coast League¡¯s American Southern Division with a 1 1/2-game lead over San Antonio. The regular season ends Sept. 2.