The team also reinstated outfielder Fernando Martinez from the disabled list and optioned him to Oklahoma City, where he was on a Minor League rehab following a concussion.
Maxwell, who has not played in the outfield since June 20 due to lingering discomfort in his ankle, will undergo an arthroscopic procedure within the next week to remove the loose bodies and will miss four to six weeks. Maxwell has eight home runs and 24 RBIs in 61 games this season with a Major League-best three pinch-hit homers.
"We thought it was something he could probably play through and it's possible that could have happened, but we really felt this was the right time to go ahead and take care of it so he could get back here and play 100 percent for whatever the balance of the season was when he gets back," Astros general manager Jeff Luhnow said. "For him and for us, that's the best answer."
Moore was expected to arrive in Houston on Tuesday night after playing earlier in the day for Oklahoma City, where he was hitting .318 with 26 doubles, 10 home runs and 54 RBIs in 73 games. He can play every position except shortstop, center field and catcher.
"It's a great opportunity for us to see Scott Moore," Luhnow said. "He's been absolutely delivering everything we've asked him to deliver in Oklahoma City, playing good defense. He's a versatile player and he's been hitting, hitting for power, taking walks, doing everything we asked him to do. It's great when you get an opportunity to promote a player who has done exactly what we've asked him to do."
Moore, 28, who was originally a first-round Draft pick of the Tigers in 2002, was with the Astros in Spring Training after signing a Minor League contract on Nov. 15, 2011. He has played in a combined 80 Major League games with the Cubs and Orioles over parts of four seasons (2006-08, 2010).
Buck went on the DL on May 29 with tendinitis in his right Achilles after appearing in 33 games for the Astros this season. He was 3-for-21 during his Minor League rehab stint with Double-A Corpus Christi, and is now healthy. The Astros have 10 days to make a decision on what to do with Buck's contract, but Luhnow said they want him to stay in the organization.
"I talked to him last night and expressed our desire to maintain him in the organization," Luhnow said. "Obviously, he's a veteran and had been outrighted before and can elect his free agency, but I'm hoping he chooses to stay with us and play every day and earn another trip back up here."
Martinez, 23, played in six games with Oklahoma City while on a rehab assignment prior to being reinstated from the DL and optioned. He had been on the DL with post concussion syndrome. For the season, Martinez is hitting .340 in 57 games for the RedHawks with eight home runs and 40 RBIs. He appeared in four games for the Astros prior to being placed on the DL on June 9.
"He was playing very well before he came up and struggled a little bit when he got here, and we need him to go back and find what he had before," Luhnow said.