It's a major blow for the Giants. The rookie had worked his way into the starting middle linebacker role in recent weeks and had an interception and sack in Sunday's 24-3 win over the Washington Redskins.
Connelly, 23, was a fifth-round pick earlier this year out of Wisconsin. He suffered the noncontact injury while attempting to make a tackle in the fourth quarter.
"It's unfortunate, but he's got a really bright future and he'll come back from this," coach Pat Shurmur said.
Connelly was wearing the headset and making the defensive play calls against the Redskins with Alec Ogletree sidelined by a hamstring injury. Ogletree and Tae Davis (concussion) could both return after missing Sunday's game.
Connelly had worked his way into the starting lineup regardless of the injuries. He had an interception in each of the past two weeks, both Giants victories, and 20 tackles total this season.
The Giants (2-2) have played better defensively in recent weeks. They allowed just six points in the last six quarters they played with Connelly in the lineup after starting the season by allowing 56 points in the first six quarters.
There is a strong chance the Giants look outside for help at linebacker. They currently have David Mayo (signed just before Week 1) and Nate Stupar (re-signed last week) as their only healthy linebackers on the active roster. Undrafted rookie Josiah Tauaefa is also an option from the practice squad.
The news of Connelly's injury comes on a day when the Giants roster did receive reinforcements. Veteran wide receiver Golden Tate, their top offseason free-agent acquisition, returned Monday from a four-game suspension for violating the league's policy on performance-enhancing substances.