The second-year pro injured his knee Sunday against the New York Jets while playing on the punt coverage team.
Mulumba tweeted the news on Wednesday morning.
1/2 Unfortunately, My ACL injury was confirmed this morning, and it's just one of those bumps on the road! Nothing that I can't overcome!— Andy Mulumba K. (@A49K) September 17, 2014
While Mulumba had not played at all on defense this season, he was a key member the Packers' special-teams units.
Last season, Mulumba made the team as an undrafted rookie from Eastern Michigan and because of injuries at the outside linebacker spot, he played 355 snaps (or 31.8 percent of the defensive plays in 2013). He recorded one sack and 34 tackles in 14 games (including three starts).
The Packers will eventually place Mulumba on injured reserve, but they have not added anyone to take his place on the roster. They don't necessarily need to add a linebacker, given that they kept 11 on their roster.