There's already a fanpost on Charlie Morton's implication that Joe Kerrigan had him scrap his sinker last year, but just to put in my two cents here: the Post-Gazette has the audio of the interview where that came out, and it's really good stuff. I highly recommend you go listen. I was starting to write a comment on the fanpost before I listened to it, and essentially what I was going to say was this:
1) It's entirely possible that Morton's rediscovery of his sinker will have little to no impact on his performance this year. This is the time of year where, if you're a team in the Pirates' position, reasons for last year's failures suddenly come out of the woodwork, especially when coaches are fired, and these reasons are used to show why last year won't happen again. Some of them might turn out to be important, but some might not.
2) Nonetheless, this particular story is interesting, in part because Morton just looked so uncomfortable last year. I don't know how important the sinker is in isolation, but in the context of Morton's repertoire, it could mean a lot. It would be like if you asked me to have a conversation with you without the words "I" and "it." It would be possible, certainly, but I would be stuttering and second-guessing myself. Morton had good stuff last year, but if he felt like he was out of his element because he didn't have one of his pitches, that might help explain why he might look great for a couple of innings and then suddenly leave a fastball hanging up in the zone for an opposing batter to crush.
Here are some quotes from the interview:
I think when I get that good sinker action, it allows me to be more aggressive in the zone. I don't have to be too fine, I can just be more over the plate, just down in the zone ...
I'm not, those pitches that I'm making for double plays [in Spring Training this year], those aren't well-placed pitches, they're just down in the zone with good action, and they're just hitting them into the ground, so it kind of leaves a little room for error ...
There's more of a sense that I'm more working on a way of pitching in one specific direction as opposed to trying to figure out what might work and what might dig me out of a hole, it's just kind of, I think I feel like I'm working on something that's me on the mound ...
Morton also talks about how he can use his four-seamer and his sinker to balance one another, and how he can rely on the sinker to generate grounders when he gets behind in the count.
Like I said, all this might turn out to be nothing. But this looks to me like a pretty compelling story about why Morton looked so confused last year, why he might throw lots of fastballs at certain times and very few at others, and why he occasionally would seem to lose it and give opposing batters complete cookies. It sounds like the sinker is the more reliable pitch for him, one he can depend on when he needs a strike. And more than that, I doubt it even has much to do with whether the sinker is, in isolation, a good-looking pitch - the way Morton's describing it, it's an important pitch because it gives him variety and makes his four-seamer harder to hit.
I'm not sure I'd blame Kerrigan too much without hearing his side of it. He was trying to coach, which is what he's supposed to do, and it's easy to second-guess him now that he's gone, especially when the season hasn't started yet. But I do hope this narrative will turn out to be meaningful once the regular season begins.