"We didn't want to rush [Fields] back," Porter said. "It's one of those things, it's a muscle. It could be two days, it could be five days, but we didn't want to rush him. We have Clemens there, and he's available and it gives us flexibility."
Fields has appeared in 28 games this year, going 1-4 with a 5.93 ERA, but in his last 14 outings, he's 1-1 with a 1.17 ERA.
"It's tight and a little sore," he said. "We'll see how it feels today, the next couple of days, and move from there. It's always frustrating when you get setbacks like this, but it's part of the game. I'm going to do whatever I can to get back as quick as I can."
Clemens has been burning up the path between Oklahoma City and Houston this year, with this marking his fourth callup to the big league club.
In 11 games for the Astros in 2014, he's pitched in 22 2/3 innings and allowed 24 hits, 13 earned runs and has walked 12 batters. He last pitched for Houston on June 7 in Minnesota, giving up four earned runs and two hits in two innings.
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.