"They took a look at it and said the shoulder wasn't moving the correct way, just due to fatigue," Butler said.
The issue is a concern because rotator cuff fatigue was the reason Butler went to the 15-day disabled list after making his Major League debut on June 6 in a loss to the Dodgers. Butler made just two more starts, and when the season ended, he was pulled from his Arizona Fall League assignment and given a strengthening program.
In a regular-season game and with more experience, Butler might have been able to continue. However, he thought it wise to report the issue to trainers and manager Walt Weiss.
"Every pitcher has to come to grips with that," Weiss said. "With the shoulder issue he had last year, it probably scared him when he felt something there. And it's Spring Training. He did the right thing."
Butler said the fatigue is not in the same area as last year, when the pain first showed up in the lat area and later was toward the back of the shoulder. He said he felt "too loose" throwing in 96-degree heat Monday.
"I'd rather throw in 30 degrees," he joked.
The issue left Butler frustrated.
"We did a lot of things this offseason, thought we were free and clear of it," Butler said.
"I didn't want to say anything. That's me. I want to be out there competing. The way it went last year and the way it's gone this year, it was better to say something to make sure I'm ready for the regular season."
Colorado released veteran righty Jhoulys Chacin earlier in camp, figuring the team had more and better options, but events of camp have shown that a team can't have enough pitching depth.
Lefty Jorge De La Rosa will miss the beginning of the season because of left groin tightness that flared early in camp. With Butler down for a couple of days, it's quite possible he won't be ready when the roster is devised.
Barring further injury, three starters are set -- Opening Day starter Kyle Kendrick, righty Jordan Lyles and lefty Tyler Matzek, who is the presumptive home opener starter. The Rockies don't need a fifth starter until April 14, so they're likely looking at one more starter.
One candidate, Jon Gray, Colorado's top prospect, struggled through his last outing, lasing two-thirds of an inning Saturday against the Cubs. Gray also reported feeling tightness in his right groin, but it's not expected to affect his next start. Other competitors are fellow righties Christian Bergman and Chad Bettis.