Being unfamiliar with hamstring injuries, he's uncertain of its severity and will await the results of his MRI on Friday.
"Somewhere in the last couple of steps, I felt something kind of pop in the back of my knee. I don't know what happened exactly," he said.
"I don't know at this point. We'll just wait and gather some information tomorrow."
Roberts was understandably down after being limited to 17 games last season and 115 over the past three years.
Asked how he felt mentally, Roberts said, "Not too great. I don't know. I don't have a whole lot of words for it right now. Frustrated, but we'll get the results and hopefully move forward.
"Never had one. I've never been hurt with a hamstring injury or anything, so we'll see what happens."
Pitcher Miguel Gonzalez expressed his concerns for Roberts.
"He's been struggling for the past three years and he's a great, great teammate," Gonzalez said. "Battles, plays hard and that's what you want in a teammate. I feel bad about him, but I just wanted my thoughts and prayers to be with him. And I hope he comes back quickly. And I think he will. He said he's hurting a little bit, but he will be back."