The Astros optioned Jonathan Villar to Triple-A following Monday night's 10-4 loss to the Mariners so Hernandez takes his place on the roster. It's unclear what other roster moves would be made to accommodate Santana and Chapman. The Astros placed outfielder Dexter Fowler on the 15-day DL with a right intercostal strain (retroactive to June 27) and designated right-handed pitcher Jerome Williams for assignment..
Santana, one of four players acquired from the Phillies in the 2011 blockbuster deal that sent Hunter Pence to Philadelphia, was hitting .304 with 13 homers and 51 RBIs through 84 games for the RedHawks. He has been playing right field. He would be the last of those four players to reach the Majors.
Hernandez went 3-for-5 with a home run and two doubles with a season-high four RBIs on Monday in Oklahoma City's 10-6 win over Colorado Springs. He is hitting .337 with eight homers and 31 RBIs in 67 games for the RedHawks.
Chapman, who appeared in six games for the Astros earlier this year, has thrown in 26 games for Oklahoma City and has allowed only three earned runs in 28 2/3 innings (0.94 ERA) while striking out 37.
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.