"We had to remove him from the game because it spasmed up on him," Porter said. "He could barely move. It locked up on him. Hopefully, he can just continue to move forward."
The injury is similar to an issue that forced the Astros to scratch Guzman from a scheduled start against Chicago on July 20.
"It was almost the same thing," Porter said. "Again, it's not like they can diagnose it beyond just a back spasm."
Further tests are expected, but Porter is optimistic this won't be a recurring issue.
"It comes and goes, because he hadn't had a problem since the time we took him out of the lineup [in Chicago], until last night," Porter said. "That's why they're going to run a bunch of tests, to rule out more causes."
Guzman was 1-for-2 with an RBI single in Friday's game before being pulled after the fourth inning. The RBI was just his eighth of the season, to go along with a pair of home runs and a batting average right at the Mendoza Line.
Chris Abshire is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.