As reliever Robert Carson heads Wednesday for an MRI on his left elbow, Terry Collins said he is "absolutely" concerned about the rookie's injury.

Carson was forced to depart Tuesday's game against the Pittsburgh Pirates in the seventh inning. The team announced Carson preliminarily has an elbow strain. X-rays at the stadium were negative, but the MRI will better reveal the extent of the damage. Carson did not speak postgame.

"When you run them out at this time of year, any of those guys, you just hope that they've kept themselves in good enough shape," Collins said about using rookie pitchers in September. "Fortunately right now it doesn't sound bad. But you always concern yourself. That's why we shut Matt (Harvey) down. That's why we shut (Josh) Edgin down."

Carson worked as a starting pitcher in the minors last season, logging 128 1/3 regular-season innings. He then tossed another 17 1/3 innings working in relief in the Arizona Fall League. This year's innings count between the majors and the minors is a relatively paltry 64 2/3, but Carson's appearance total has jumped dramatically -- to 58, from 35 last season including the AFL duty.

Carson likely is done for the season, joining Frank Francisco (elbow) as well as the shut-down Harvey and Edgin in that category and leaving Collins wondering if he has enough arms to get through the season.