Chacin said he believes the issue is minor, and the problem is more in his biceps than the shoulder, but the MRI will check the biceps tendon, which begins high in the shoulder and runs along the arm, and the rotator cuff muscles.
"I think it's not too bad," Chacin said. "I'll just take a couple of days to get the inflammation and the pain out, then work to get back throwing again.
"I don't feel it when I'm on my throwing program, throwing real easy. When I start to throw hard, that's when I feel it."
Rockies manager Walt Weiss insisted Sunday that the club will not rush Chacin, although he is a major part of the potential rotation. Chacin, 24, went 14-10 with a 3.47 ERA in 31 starts and threw a career-high 197 1/3 innings last season, and is being counted on as half of a 1-2 punch with lefty Jorge De La Rosa, who went 16-6 with a 3.49 ERA last season.
"I feel good about the fact that we caught it early. He's had it at times in the past, not necessarily last year, but the year before," Weiss said. "So we're being really cautious. We've got time on our side. We'll hope for the best, but I'm not overly concerned right now, just because it's February."
Chacin didn't report any problems throwing this winter, and he went to the Rockies' complex in the Dominican Republic to complete his pre-Spring Training work. Weiss said there may have been soreness at the end of Chacin's time in the Dominican.
Chacin missed most of 2012 with a right pectoral nerve issue that was a mystery for the Rockies for two months until a specialist made the determination. But Rockies head athletic trainer Keith Dugger said Chacin feels much better now than then, and does not believe the current problem is related to the pectoral nerve issue.
"He's a little sore in the biceps tendon and kind of all over … inflammation, you would think."
While the Rockies, looking for a turnaround after last-place finishes in the National League West the last two years, can ill afford to lose one of the NL's emerging hurlers, they entered camp with depth.
The Rockies traded with the Red Sox for lefty Franklin Morales, received righty Jordan Lyles in a trade with the Astros and have lefty Christian Friedrich healthy after missing a year with a back injury. They were brought in to compete with hard-throwing but inconsistent righty Juan Nicasio, but also offer depth in the case of injury.
Morales, who broke in with the Rockies as a starter in 2007 but has pitched mostly out of the bullpen since, said he didn't know Chacin's status. But Morales is prepared for what the Rockies need.
"I came to do my job and pitch, and I like to be ready for any situation."
Rockies pitching coach Jim Wright said of Chacin, "We know what he means to the team, and we just hope it's a minor issue that can be addressed with rest, and we can get him ready."