For discussion, here's Faulk's game logs:
22/128 vs. ARI (5.8 YPC - AZ gave up an average of 4.7)
12/20 vs ATL (1.7 YPC - ATL gave up an average of 3.9)
12/44 vs NO (3.7 YPC - NO gave up an average of 4.7)
23/121 vs SF (5.3 YPC - SF gave up an average of 4.0)
15/51 vs SEA (3.4 YPC - SEA gave up an average of 4.5)
15/40 vs TB (2.7 YPC - TB gave up an average of 4.1)
12/61 vs MIA (5.1 YPC - MIA gave up an average of 4.3)
12/66 vs NWE (5.5 YPC - NWE gave up an average of 3.9)
18/139 vs SEA (7.6 YPC- SEA gave up an average of 4.5)
13/6 vs BUF (.5 YPC - BUF gave up an average of 3.6) <--Knee Bruise
7/7 vs GB (1 YPC - GB gave up an average of 4.6)
MISSED GAME
MISSED GAME
10/22 vs ARI (2.2 YPC - AZ gave up an average of 4.7)
17/54 vs PHI (3.2 YPC - Philly allowed 4.3 but had clinched homefield)
7/15 vs NYJ (2.1 YPC - NYJ allowed 3.6)
Since Faulk shared time off and on with Jackson throughout the year, I used yards per rush as a statistic. Green = more than .5 YPR better than the team's YPR allowed, Red = more than .5 YPR worse.
Before the knee bruise, he was putting up decent yards per carry more often than not (5 good games, 4 bad). After the knee bruise, he put up all bad games. His best games were nothing spectacular, three 100 yard games out of his first nine, although in the interests of not getting called cheesy by HS, I left his 50 receptions for 310 yards out of the discussion; he had a 135 total yard game against Miami.
(With regards to Faulk's bad stretch of goal line carries, you can see above that they happened against the two toughest Ds he faced, New England and Buffalo)
Looking at the above chart, I don't see a guy who is completely done. I see a guy who put up decent (but unspectacular) numbers until he got hurt. It makes sense that they don't want to overwork him, because he has had problems with that knee before. But I highly doubt that means he's a <5 carry a game backup.
By the way, here's the numbers for Jackson:
7/50 vs. ARI (7.1 YPC - AZ gave up an average of 4.7)
3/10 vs ATL (3.3 YPC - ATL gave up an average of 3.9)
2/15 vs NO (7.5 YPC - NO gave up an average of 4.7)
10/46 vs SF (4.6 YPC - SF gave up an average of 4.0)
5/64 vs SEA (12.8 YPC - SEA gave up an average of 4.5)
13/48 vs TB (3.7 YPC - TB gave up an average of 4.1)
6/27 vs MIA (4.5 YPC - MIA gave up an average of 4.3)
3/1 vs NWE (.3 YPC - NWE gave up an average of 3.9)
10/47 vs SEA (4.7 YPC- SEA gave up an average of 4.5)
7/29 vs BUF (4.1 YPC - BUF gave up an average of 3.6)
8/40 vs GB (5.0 YPC - GB gave up an average of 4.6)
26/119 vs SF (4.6 YPC, SF gave up an average of 4.0) <--Knee Bruise
MISSED GAME
MISSED GAME
24/148 vs PHI (3.2 YPC - Philly allowed 4.3 but had clinched homefield)
10/29 vs NYJ (2.9 YPC - NYJ allowed 3.6)
7 well above average games, 4 well below average games, and three slightly above average games, on a yards per carry basis. That's a little better than Faulk's 5 above average games and 4 below average games against mostly the same defenses, although the very small sample size has skewed some of them (I highly doubt he'd have put up 12.8 yards per carry against Seattle, or 7.5 against New Orleans, or .3 against New England, in extended duty).
To me, I see Jackson as a guy who played a little better than Faulk last year, and earned the right to be the lead back in a running back by committee. He may even have a good run down the stretch if Faulk gets hurt, but with Faulk seeing fewer carries, it may not be as likely that Faulk gets hurt. I don't, however, see him as a guy who massively outperformed a healthy Faulk. I think that's where the disagreement comes, because HS and PB think he's the vastly better back, while I think Faulk wasn't that bad.