Tommy Kelly tore his ACL in week 5. That gives him 8 to 9 months to rehab until the start of training camp, right?
AFAIK, Kelly's injury was just listed as "knee". I haven't seen any reports that it was specifically an ACL tear, but I'm skeptical that it was anything that severe since the Pats kept him on the active roster for another 4 weeks before sending him to IR.
BTW, the 2013 DT Carousel Timeline went like this:
Week-2 - CHRIS JONES claimed off waivers from TBY (who had claimed him off waivers from HOU during final cuts the week before)
......
Week-4 (@ATL) 9/29 - WILFORK, Achilles - Surgery on 10/1, sent to IR 10/3.
Week-5 (@CIN) 10/6 - KELLY, "knee injury"
------ 10/9 - DT ANDRE NEBLETT signed
------ 10/12 - FORSTON called up from PS; Neblett cut
Week-6 (NOL) 10/13 - MAYO, torn pec - Surgery and sent to IR 10/16.
Weeks 6-9 - JONES & VELLANO START
Week-8 10/23 (before home game vs. MIA) - SILIGA signed to PS
Week-9 10/29 (before home game vs. PIT) - SOPOAGA acq. from PHL, plays vs. PIT
------ 11/2 (day before PIT game) - KELLY sent to IR
Week-11 (@CAR) - Sopoaga starts
Week-12 (DEN) - Sopoaga starts
Week-13 11/26 (before game @HOU) - SILIGA called up from PS and plays in game; Forston waived back to the PS
Weeks 14 & 15 - SILIGA STARTS, Sopaga active
Weeks 16 & 17 - SILIGA STARTS, Sopaga is healthy scratch
My guess is that the initial diagnosis for Kelly (in Week-6) was that he might avoid surgery and be able to come back for the last couple games and the playoffs, so the Pats kept him on the 53. However, by Week-8, he wasn't making much progress and the run-D was sucking with only Jones and Vellano (and without Mayo), so they started making roster moves to try to cover - and the acquisition of Sopoaga essentially bumped Kelly to IR in Week-9.
After Week-12 (DEN), though, it became clear that Sopaga wasn't what they'd hoped. Meanwhile, Siliga had made significant progress on the PS under BB's personal tutelage, so they gave him a shot (over Forston). By the end of Week-15, it was clear that their best three DTs by far were Siliga, Jones and Vellano, so Sopoaga sat the rest of the way.