There’s bad days, and then there’s what Sergio Romo had against the Mariners in the midst of an 18-3 thrashing on Saturday. Romo faced five batters in the eighth inning, and you can imagine the type of opposition you face in the eighth inning of a Cactus League game on March 8:

I could do the same for his first outing of the spring, when he allowed six runs in an inning to Oakland, but the point here isn’t really to go on a tour of the lower levels of the American League West. The point is that there’s a sizable portion of you, I imagine, who have heard of zero of those five names. And yet Sergio Romo, World Series closer, All-Star, among the best relievers in the game for the last five years, managed to retire exactly none of them. After four games, Romo has faced 23 batters, allowed 14 of them to reach, and 12 to score (11 earned).

So… panic, right? Even within the context of “spring numbers don’t matter”, because no quality big leaguer should have such trouble with a collection of names like that without hiding some kind of serious injury.

It’s hard to spin those kind of results, but if there’s a good explanation for it, Romo seems to have it. He’s refusing to throw his slider:

Sergio Romo hasn’t yet unveiled his slider to opposing hitters this spring. “It’s not like I haven’t been practicing it or anything,” Romo said Sunday. “I don’t need to throw it in a game for me to sit there and know that I’ve got it.”

No sliders! If you know Romo for anything, it’s for his slider. A Romo without his slider is hardly a Romo at all; of the 304 pitchers over the last two seasons to throw at least 100 innings, exactly one — Luke Gregerson — has thrown a higher percentage of sliders than Romo’s 54.9 percent. If you buy into pitch values, his slider has been the second-best of all relievers over the last two years, behind only Adam Ottavino, of all people.

Or, since everyone loves a good graph:

The slider is in red. Romo loves his slider. Instead, Romo has been throwing only fastballs and changeups, hoping to add a usable third pitch into what has been a career made up of approximately 95 percent fastballs and sliders. As he put it, regarding the change, “that’s the reason why I’m trying to use it — so that I can be more of a complete pitcher.”

It’s not really hard to see why, either. Romo was very, very good in 2013, but he was somewhat less good than he’d been in previous years. After swinging-strike rates of 17.0 percent (2011) and 15.3 (2012), it was down to 13.6 last year, and he had difficulty finishing hitters off, as his K/9 rate not only fell below 10.16 for the first time since his 2008 rookie season, it plummeted all the way to 8.65. Lefties in particular began to catch up with him; he’d allowed wOBA of .262 (2011) and .221 (2012) previously, then .319 last year. It should come as no surprise that the changeup is a pitch he wants to master almost entirely to neutralize lefty hitters.

When I say “entirely,” I’m not exaggerating. Romo threw the change 62 times in 2013. 61 of them came to lefties, and I’m willing to bet the lone offering to a righty was either miscategorized by PITCHf/x or was merely an attempt to completely confuse Mark Ellis. Even then, it was merely a “show-me” pitch. 36 of the changes, or 57.6 percent, were balls. It was merely to remind lefties that it existed more than it was to get them out, but as Romo enters his age-31 season with a weakness in his game, he’s taking the opportunity to attempt to improve that weakness. It says, I imagine, a lot about Romo’s standing with the team and with manager Bruce Bochy that he’s even able to experiment like this. Pitchers with concerns about their jobs need to focus on looking good. Pitchers with solid standing can play with new toys.

While Romo would like to improve his arsenal, the Giants badly need him to flip the switch back to his old self once the season starts, because you might be surprised to find that what was once a strength is now a pretty big source of concern. San Francisco’s bullpen ranks above only the Mets and the Brewers on our depth charts, and that’s because the days of Romo setting up for Brian Wilson are long gone. Santiago Casilla turns 34 this year, and while he’s shown an ability to outpitch his FIP over the last few seasons, I’m taking the “over” on another 2.16 ERA after a season in which his strikeouts dipped and his control issues persisted. Javier Lopez is a fine LOOGY, but 75.1 innings over two years means his impact is limited; Jeremy Affeldt, 35 in June, is coming off a poor year marred both by a groin injury and a nearly 1:1 strikeout-to-walk ratio.

And those are just the four guys with guaranteed jobs. Behind them is a cast of thousands, as this MLB.com article lays out nicely:

If there’s concern here, it’s not that Romo looks awful, or that he’s been getting lit up by low-level players with little chance of making a big league impact, because it’s actually a good thing that he’s attempting to improve his game. It’s just that it shows more than ever how badly the Giants bullpen is going to rely on him, and how much they need him to stem the slight performance slide we’ve seen over the last two years. Another weapon against lefties would certainly help with that.