After sitting out the first month of the season because of a right shoulder strain that showed up at the beginning of Spring Training, Chacin has made 11 starts, but he hasn't found his top-end arm strength. His fastball velocity is down and his pitches lack sink. Those issues have led to a 1-7 record and 5.40 ERA, and increasing difficulty recovering from start to start.
"He's feeling a little worse," Rockies manager Walt Weiss said. "It's been tougher for him to pitch through that. Early on, it felt like there was some rust there and it took him a couple innings to get loose.
"Now it doesn't feel as good in between starts, so he's having to alter his throwing program. This happened the last couple of starts. It's escalated to the point where we needed to do something about it."
The team announced that left-hander Franklin Morales will start in place of Chacin on Thursday. Because Morales was in the rotation to begin the season, the number of starters the Rockies have used this season will stay at 12, and the team will have an all left-handed rotation. Colorado recalled right-handed reliever Chad Bettis from Triple-A Colorado Springs to take Chacin's 25-man roster spot.
The Rockies are in Washington for a three-game series against the Nationals, but Chacin flew back to Denver for an MRI, which occurred Monday afternoon. Results are expected on Wednesday. Head athletic trainer Keith Dugger said the focus is on the shoulder muscles. In 2013, Chacin had a mysterious loss of power that, after multiple examinations, was revealed to be a nerve issue. But Dugger said the team is expecting this to be a muscular issue.
Rockies lefty starter Jorge De La Rosa said even with the team's struggles, he was happy Chacin spoke up about how he felt and put safety first.
"He wasn't feeling good, since the first game I saw him pitch," De La Rosa said. "He's a warrior and he wants to pitch. He has a lot of heart. Thank God he talked and said he didn't feel good. He needs to get healthy and get back stronger."
Chacin joins left-hander Brett Anderson (broken left index finger), and right-handers Tyler Chatwood (right flexor tendon strain), Jordan Lyles (broken left hand), Christian Bergman (broken left hand) and Eddie Butler (right rotator cuff strain) on the DL.
The pitching injuries are a contributing factor to the Rockies having won two of their last 15 games going into Wednesday's finale against the Nationals.
"When your pitching gets tested, injury-wise, that's always a challenge," Weiss said. "It's easier to overcome injuries to your position players than it is to your pitching depth."
The Rockies, 11 1/2 games behind the National League West co-leading Giants and Dodgers going into Wednesday, are left hoping no one takes control of the division.
"Just think about those [winning] teams -- I can name all five [starters]," Corey Dickerson said."I couldn't name our five starters. That's kind of tough.
"If we can just get some wins and don't get too far where you can't battle back, every team is going to hit a [bad] stretch. That's just baseball."
In addition to the half-dozen starting pitchers, also on the disabled list are lefty reliever Boone Logan (left elbow inflammation), left fielder Carlos Gonzalez (surgery to remove a tumor from his left index finger), third baseman Nolan Arenado (fractured left middle finger) and first baseman-outfielder Michael Cuddyer (left shoulder fracture).
Logan and Arenado recently began injury rehab assignments at Colorado Springs, and Anderson is due to begin his rehab assignment Wednesday.