Jets OLB Josh Martin has been placed on injured reserve after suffering his second concussion this season in Sunday's loss to the Jaguars.
Martin made his season debut on Sunday after missing the team's first three games with a concussion he sustained in the preseason. He left Sunday's game and did not return.
Jets coach Todd Bowles called the concussions "definitely alarming" and "a concern" for the 26-year-old linebacker.
DE Bronson Kaufusi was activated from the practice squad to take Martin's place on the roster and LB Harvey Langi was signed to the practice squad.
Kaufusi, a former third-round draft pick, played in three games with the Ravens last season, including one start. He was waived by the Ravens on Sept. 1 and joined the Jets' practice squad two days later.
Langi signed by the Patriots last year as an undrafted free agent but was in a serious car accident last October and suffered multiple injuries. He was released by the Patriots on Sept 1.
Martin went undrafted and spent time with the Chiefs, Buccaneers, and Colts before signing with the Jets' practice squad in 2015. He played in 14 games last season, including nine starts, and registered 20 tackles and 1.5 sacks.