Browns linebacker D'Qwell Jackson injured his right pectoral muscle during practice Tuesday evening, the team confirmed Wednesday afternoon.
Last year, Jackson tore his left pectoral, had surgery to repair it and missed the final 10 games of the season.
Neal Gulkis, the Browns' vice president of public relations, said more tests must still be done on Jackson to determine the severity of his injury.
"There's nothing definitive at this point," Gulkis said.
Since Browns training camp opened to the public July 31, Jackson had been practicing with the first-team defense as an inside linebacker. After Jackson suffered his injury Tuesday, linebackers Eric Barton and Kaluka Maiava got reps with the first-team defense.