Nicasio suffered a neck injury when he was struck in the second inning by a ball hit by Ian Desmond. The ball slammed into the right side of Nicasio's head with a smack and caromed away, rolling over the first-base line.
The right-hander went down and lay flat on his back on the mound for several minutes as he was attended to by training staff. He was moved onto a cart by four medical personnel, helped by teammate Todd Helton and manager Jim Tracy.
He could be seen blinking and moving his hands and legs before he was put on a stability board and loaded onto a cart.
Nicasio (4-4) was taken to the hospital where the team said he was resting comfortably and undergoing tests.
"It's hard to see another ballplayer get hurt," Desmond said. "I hope he's OK."
Rockies shortstop Troy Tulowitzki said the team had trouble focusing after the injury occured.
"We're obviously very worried," Tulowitzki told InsideThe Rockies. "We were asking (about him) in the dugout, even during the game. We know he has some kind of a neck injury. We don't know to the extent or anything. We kept asking questions as the game was going on. That's all we really know. That's all we've been told. Our thoughts and prayers are with him."
The Associated Press contributed to this report.