After an MRI later Saturday confirmed the damage, he is expected to miss approximately three more weeks because of the strain.
Beltre was running bases for the second consecutive day, and during the middle of his workout, he rounded first base and stopped five or six steps later.
"It just happened in the process," manager Ron Washington said. "Hamstrings are fickle. You have to go through the baseball activity to check it. You have to start, stop, move."
After feeling something in the hamstring, Beltre walked into right field clearly frustrated, talked briefly with athletic trainer Jamie Reed and stood with his hands on his hips for a few minutes before walking to the dugout, ending the workout early.
"You would be frustrated too if you felt like you were ready to go and you realized that the hamstring says, 'I'm not,'" Washington said.
Beltre was placed on the disabled list July 23 and was eligible to come off on Sunday. But he was expected to go through more workouts and then go out for a rehab assignment before being activated. That timetable has now been delayed three more weeks.
Beltre, 32, was batting .276 with 20 homers and a team-high 76 RBIs at the time of the injury. Since he's been out of the lineup, the club is 6-7.
Richard Durrett covers the Rangers for ESPNDallas.com.