Jackson is recovering from a similar injury that ended his 2009 season after just six games. The Plain Dealer reported that the extent of Jackson's latest injury is unknown, but it isn't to the surgically repaired muscle.
According to the Plain Dealer, Jackson slowly walked off the field midway through practice and had his head down. He later returned to the field with a towel wrapped around his neck, and his teammates came over to seemingly console him. The newspaper reported that it appeared Jackson pointed to his chest at one point during those discussions.
Jackson, who was a restricted free agent, expressed his desire for a long-term contract this offseason, and he even skipped a mandatory minicamp to show his displeasure, saying at the time that "I still want to retire as a Cleveland Brown, but I have to be protected." But Jackson relented just before the June 15 deadline and signed his one-year, $1.759 million tender.
Jackson, who's entering his fifth NFL season, was the Browns' leading tackler in 2008 with 154. He had 59 tackles in his six games last season.
Jackson's apparent injury was the second bit of bad news for the Browns on Tuesday. Starting cornerback Eric Wright was kept out of the night practice after injuring his right leg during the morning workout.
Wright was hurt during 7-on-7 drills about halfway through the first of the Browns' two practices. Trainers attended to Wright on the sideline, then put an ice wrap on his leg. He limped to the team's training facility for more treatment.
Wright's injury appeared to be to his hamstring. It's not yet known if it will keep him out of Saturday's preseason opener against the Green Bay Packers.
Before the evening session, Wright walked onto the field without a limp but didn't have his helmet. He worked out on a hand bike as his teammates warmed up nearby.
A second-round draft pick in 2007, Wright had four interceptions last season. If he is out for any extended period, rookie Joe Haden likely would take his spot.
Haden is excited about the possibility of more playing time.
"Real excited," said Haden, the seventh overall pick this year. "It definitely means a whole lot more playing time and that's just more time to make plays. The only way that you make plays is out on the field. If I get a whole lot more time, I'm looking forward to that."
The Associated Press contributed to this report.