"His back locked up and he doesn't look too good right now," Weiss said of Anderson, who was unavailable after the game. "We've lost a lot of pitchers this year due to injury and it looks like we'll go down that road again."
Anderson threw just one strike to Cubs center fielder Arismendy Alcantara to start the fourth inning before things took a turn for the worse.
"It came on all of a sudden," Weiss said. "He hit the double the inning before. I don't know if that contributed to it at all, but after he threw that pitch it just locked up on him."
Weiss said Anderson's warmup pitches that inning were fine, but once the spasms began it was quickly apparent he wouldn't be able to continue, and he was escorted off by a trainer.
Anderson had a one-hitter going through his first three innings, striking out two without issuing a walk. He threw 33 pitches, 25 for strikes.
Entering Monday, Anderson had compiled a 2.84 ERA and 22 strikeouts in his four starts since returning from the broken finger that forced him to miss three months.
"Brett has been real good particularly since he came back from the DL," Weiss said. "I feel like he's given our rotation a lift, the way he operates, the tempo he pitches and the professionalism he brings to pitching in general. He really does a lot for us. It looks like we're going to lose him here for a little bit."
While a laundry list of seemingly never-ending injuries has prevented Anderson from throwing over 100 innings in a season since 2010, this back injury is something new as far as Weiss knows.
Left-hander Franklin Morales, who came on to relieve Anderson Tuesday, was recently moved back to the bullpen due to Jordan Lyles' anticipated return Wednesday but should be the Rockies' top option to replace Anderson.
A more long-term solution could be right-hander Christian Bergman, who has already pitched two rehab starts for Double-A Tulsa as he works his way back from a broken non-pitching hand. Bergman isn't eligible to return from the 60-day disabled list until Aug. 20, though, so the Rockies will need someone else to fill in for Anderson in the meantime.