HOUSTON -- Astros ace Bud Norris left Tuesday's game against the Kansas City Royals due to tightness in his back.
Norris blanked the Royals through six innings, allowing five hits, no runs and striking out two.
"We're only going to push him so far," said Astros manager Bo Porter. "During that long [sixth] inning, he tightened up and got stretched out, but I think we made the right decision getting him out of the game, especially since he has an extra day of rest before his next start."
He initially came back out to pitch the seventh inning, but Porter and head athletic trainer Nate Lucero visited the mound after his warmup pitches and Norris was removed for reliever Travis Blackley.
"He said [his back] was tightening up a little bit, and that's all I needed to know to go ahead and make a decision to take him out," Porter said.
Norris was originally slated to pitch on Sunday, but Porter pushed the right-hander's start back two days because of lower back issues that took Norris out of last Monday's outing against the Tigers.
With his six innings of shutout work, the 28-year-old has now allowed just eight earned runs in six home starts this season.
Blackley promptly surrendered two runs in the seventh inning after replacing Norris.
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter. Chris Abshiret is an associate reporter for MLB.com This story was not subject to the approval of Major League Baseball or its clubs.