"There is much improvement," Marlins general manager Dan Jennings said Friday. "He completed light core exercises and rode the bike with no issues."
In a week, an X-ray will be taken and Kolek will be further evaluated.
Kolek felt a sharp pain in his lower back on the right side while pitching for the Gulf Coast League Marlins on Wednesday in Jupiter, Fla. The 18-year-old faced four batters that night, and he was lifted in a game that was suspended due to weather.
"It was one of those, when it hit him, the kid felt like he was stabbed," Jennings said. "So they just took him out."
A native of Shepherd, Texas, Kolek is a hard-throwing right-hander whose fastball reached as high as 102 mph in high school.
"He's feeling much better," Jennings said.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter