Asante wasn't one of the worst CBs in the league last year. Pro Football Focus had him like 57th out of 110 CBs, which is right in the middle of the pack. Again, it was Asante's worst season and he was still average by league standards. His coverage grade was -2.0, and Alford's was -3.1. His run stop grade was -0.2, while Alford's was -2.4.
Advanced NFL Stats metrics are based largely off how many positive plays a player has (and in regards to Win Probability when those plays occur), which Asante didn't have a lot of last year. But that doesn't mean he was bad. Like I said, he didn't have his normal level of production, but when you watch tape there's no reason to think that he won't be significantly better this year. Asante is a player that thrives on being in the right place at the right time, and in the case of 2013, he didn't find himself in those situations very often. If you're going to source some advanced metrics, try to understand the methodology first.
And in one sense you might claim that sort of playing style is random and thus it's just as likely that Asante could have another bad year as he could have a good year. But then when you factor in that Asante has had a knack for being in position to make those plays for 10 years, there is every reason to believe that 2013 is an aberration, a statistical outlier in regards to the previous year. Again, because the things that Asante does well aren't things that go away with age. It's the same as what I said last year with why Grimes was likely to have several more good years left in him because he like other corners like Charles Woodson and Champ Bailey are technically sound and play smart football, which doesn't diminish with age as opposed to corners like Dunta Robinson or DeAngelo Hall who rested mostly on their superior athleticism throughout their careers, and that quickly subsides once they hit 30.
In the case of Asante, because of his playing style, he's not likely to be significantly worse at age 33 than he was at age 30.