Oswalt on Wednesday returned to Houston, where he was to be examined by a back specialist.
One of baseball's best pitchers this decade, Oswalt has had back problems before, including a stint on the disabled list in 2006 for a mid-back strain.
Wright allowed three runs in 2 1/3 innings and got his first career hit before he had to be rushed to Northwestern Hospital. Team officials originally thought he might have had appendicitis because he had complained of discomfort in that area of his body.
The Astros said Wright was staying at the team hotel Wednesday but probably would be back with the club for Thursday's series finale.
Brocail replaces LaTroy Hawkins, who went on the DL Tuesday with shingles. Maysonet was sent down to make room for right-hander Bud Norris, who was called up after Tuesday's game.