Kelly injured the knee in last Sunday's 13-6 loss to the Cincinnati Bengals. This leaves the Patriots without their two opening-day starters at defensive tackle, as Vince Wilfork is on season-ending injured reserve with a torn Achilles.
That leaves them with their third-, fourth- and fifth-string options at defensive tackle -- rookies Joe Vellano and Chris Jones and four-year veteran Andre Neblett, who was signed Wednesday.
On Neblett, Patriots coach Bill Belichick said, "He has some experience, played at Carolina and Tampa in preseason; we played against him. He has good playing strength and he's played a decent number of plays over the past three years, couple hundred plays a year, so he has some experience."
Belichick said the 6-foot, 310-pound Neblett has played both three-technique (outside shade on guard) and nose tackle.
The Patriots also ruled out veteran running back Leon Washington (ankle), while tight end Rob Gronkowski (back/forearm) is officially questionable.