Nate Irving might have played his last snap for the Broncos.

The team’s starting middle linebacker through the first eight games and an unrestricted free agent after this season, Irving was placed on season-ending injured reserve Thursday.

Irving suffered a partially torn medial collateral ligament in his right knee in the fourth quarter of the Broncos’ 43-21 loss Nov. 2 at New England.

The Broncos didn’t originally place Irving on season-ending IR because they needed to wait for the swelling around the knee to go down before they more conclusive tests could be taken. Apparently, more time revealed Irving’s injury was serious enough that he could not return in time to help the Broncos through the final seven weeks of the season, not when it left them short-handed on special teams.

The Broncos made the move Thursday after they signed rookie linebacker Todd Davis off waivers from New Orleans.

Davis was undrafted this season out of Sacramento State and spent most of the regular season on the Saints’ practice squad. He played in three games for the Saints, making two special teams tackles, but was waived Wednesday.

While Davis adds depth and special teams help, the Broncos are expected to continue to use Steven Johnson as their starting middle linebacker with fifth-round rookie Lamin Barrow also competing for the job.

Mike Klis: mklis@denverpost.com or at twitter.com/mikeklis