When Sergio Romo’s slider swoops across home plate, it’s as unhittable a pitch as any in baseball—even moving at velocities in the 70s.

It is the main reason why Romo has been so successful as a relief pitcher with the San Francisco Giants the last few years and why he so successfully filled the closer role down the stretch this past season.

But there are concerns about whether he can close full-time. General manager Brian Sabean has expressed as much. Listed at 5'11" and 190 pounds, Romo is small for pitcher, though it is not unprecedented for an average-sized man to thrive on the mound. Billy Wagner is a prime example.

Size doesn't necessarily have to be an issue, nor should it matter that Romo doesn't have a blazing fastball to overpower hitters. His junk is plenty daunting for opposing hitters. His track record gives every indication that he is a highly effective reliever, as his five-year career numbers are impressive: 2.20 ERA, 0.88 WHIP, 10.7 K/9.

A move to closer would only fail if his nerves got the best of him with the added responsibility. But based on his recent effectiveness in critical moments, Romo has the mentality and competitiveness demanded of pitchers taking the ball in the ninth inning.

Something worth considering is that, as beloved as Brian Wilson was as San Francisco’s closer, he wasn't nearly as dominant as one might believe. In four seasons closing games, he was great for one of them (2010), good for two and barely passable in the other.

With a 3.21 career ERA and a high walk rate, Wilson has given up a full run more per inning than Romo has while with the Giants. So unless Romo contracts the same aversion to closing that Santiago Casilla did last season or if his body breaks down as some fear, he should be considerably more effective than Wilson was on a year-to-year basis.

Now, Romo hasn't closed for a complete season, so no one can say with certainty that he can do the job. He just hasn't proven it yet. But closers step up all the time in the major leagues without having proven themselves.

The other concern is that Romo has never carried a heavy workload over the course of a season. He rarely throws more than one inning, and many of his appearances are of the one- or two-out varieties.

Of the 13 National League pitchers who saved more games than Romo did in 2012 and who didn't miss significant time due to injury or ineffectiveness (e.g. Carlos Marmol), none pitched fewer innings than Romo. While he threw 55.1 innings, the top closers generally pitched 10 to 17 more innings than that.

If Romo were asked to increase his load by, say 10 innings, could he handle it? What if he had to regularly close back-to-back games? He may have answered these questions in October, when he pitched 10.2 innings in the playoffs and looked even more dominant than in the regular season. Against the Detroit Tigers, he saved three games in four days.

So, yes, Romo can be San Francisco’s guy in the ninth inning, at least in 2013. Manager Bruce Bochy may want to monitor his workload, perhaps by using Javier Lopez or Jeremy Affeldt to occasionally close games. But as long he stays healthy, Romo has the skills and intangibles to be one of the National League’s best closers.

Whether Romo can fill the role beyond 2013, however, is a complete guess. The closer position is one of the most volatile in baseball, and as such, “long-term” has a unique meaning for the position. Will he be Mariano Rivera and ease the Giants of thinking about the position for the next decade? Hardly anyone has ever done that.

But unless Romo’s body falls apart soon, there is good reason to believe he will handle the role for a few seasons.