Jackson was in the midst of a comeback after missing Cleveland's final 10 games last season with a torn pectoral muscle that required surgery. He injured his other chest muscle during practice on Tuesday, team spokesman Neal Gulkis said.
According to the Cleveland Plain Dealer, Jackson suffered a strained right pectoral muscle in his chest and will miss 4-8 weeks. He's also getting a second opinion.
Gulkis said Jackson has undergone testing and will have more procedures to determine the extent of his new injury. The Browns are not expected to release any results or provide further information until Thursday, when they will also have two practices.
Agent Brian Mackler said he was waiting to hear from doctors and had no update.
Denver star linebacker Elvis Dumervil, the NFL's sacks leader last year, tore a pectoral muscle during camp and is likely to be out for five months.
Browns coach Eric Mangini gave his players a day off from practice on Wednesday, but they reported for meetings, treatment and workouts in preparation for Saturday's exhibition opener in Green Bay against the Packers.
Jackson, one of the Browns' most well-liked and respected players, left the field about halfway through the team's second workout on Tuesday. He walked dejectedly toward the locker room and favored his right side as he tried to open a rear door to the team's facility.
He returned without his pads and with a towel draped around his neck to watch the final 30 minutes. Jackson was consoled by teammates and coaches as he explained his latest injury.
Jackson tore his right pectoral muscle on Oct. 18 when Pittsburgh Steelers tight end Heath Miller landed on him. He was placed on injured reserve two days later and missed the last 10 games of his fourth pro season.
Losing Jackson, a co-captain last season, would be be a tough blow for the talent-thin Browns. However, the team has more depth at linebacker after signing free agent Scott Fujita and acquiring Chris Gocong in a trade. They have five other linebackers with experience as NFL starters.
Jackson, who led the league with 188 tackles in 2008, had a tumultuous off-season. He had threatened to hold out of camp after the Browns failed to give him a long-term contract, signing him to a one-year, $1.76 million tender for this season.
He skipped offseason practices and minicamps, but came home from a goodwill trip to Africa with a new perspective on his situation.
Information from The Associated Press was used in this report.