HOUSTON -- The Astros on Thursday announced All-Star relief pitcher Ryan Pressly will be out four to six weeks following arthroscopic surgery on his right knee, to be performed Friday in Houston.

The move is a big blow for the Astros, who placed Pressly on the injured list for the second time this year and recalled right-hander Joe Biagini from Triple-A Round Rock. The club also purchased the contract of infield prospect Abraham Toro and added him to the Major League roster, and optioned infielder Jack Mayfield back to Triple-A.

¡°You lose an All-Star reliever, it¡¯s a tough blow for the bullpen and for our team,¡± Astros manager AJ Hinch said. ¡°Quite honestly, I¡¯m glad we have answers. We¡¯ve been battling something similar for a few weeks -- sometimes getting the good version of Pressly and sometimes getting the sore version of Pressly. It got to the point we were seeking answers.¡±

Astros president of baseball operations and general manager Jeff Luhnow wouldn¡¯t say exactly what type of procedure Pressly will undergo but called the surgery minor. Pressly wasn¡¯t available for comment.

¡°It¡¯s a common surgery,¡± Luhnow said. ¡°It¡¯s not really that major of a deal. A lot of athletes have it across multiple sports. Since some players get back quicker than others, I think our conservative estimate is four to six weeks, but I think we¡¯ll see him doing activity before that.¡±

Pressly was on the IL from July 30-Aug. 8 with a right knee bruise. Pressly has a 2.50 ERA with 65 strikeouts in 50 1/3 innings but hasn¡¯t been the same since coming off the IL -- four earned runs and six hits in six outings. He gave up a tiebreaking grand slam in the bottom of the eighth inning Aug. 14 against the White Sox and came out of that game with a swollen knee.

Pressly appeared to be making progress physically during last weekend at Oakland but reported continued soreness when the team returned home this week.

¡°We¡¯re lucky this is next week or the week after, where we have at least a shot at getting him back healthy for the rest of the season, and, hopefully, we have a long October ahead of us,¡± Hinch said. ¡°We have to adapt and adjust. That¡¯s the way it is.

¡°We have a number of bullpen guys who are throwing well. I¡¯m going to challenge our bullpen to absorb the innings we had been giving Press and hope for a speedy recovery.¡±

Pressly has been used primarily as the Astros¡¯ setup man to get the ball to closer Roberto Osuna. Will Harris has had a terrific season, as well, and could get more opportunities at the back end of games. Hinch prefers to use Harris in jams with runners on base.

With only one Trade Deadline this year (July 31), the Astros won¡¯t be able to make a deal for bullpen help.

¡°The reality is you can¡¯t replace an All-Star reliever at this time of year even with the old structure,¡± Luhnow said. ¡°It¡¯s hard to do. I really do have confidence in our guys. We just got [Brad] Peacock back, Biagini is coming back now. I believe our guys will step up and rise to the occasion.¡±