Who is the Nationals' sixth starting pitcher at this point?

We conduct a poll every spring, asking Nationals beat reporters to make a bunch of predictions for the upcoming season, and the results are always fun to track throughout the year.

One of the most interesting predictions, I think, is guessing who the Nats' sixth different starting pitcher will be in a given season. Basically, it's trying to predict when one member of their opening rotation will go down and who will take his spot.

And that could be an awfully tricky thing to predict for 2017.

Who's the Nationals' No. 6 starter? They don't really have one at the moment, having taken two candidates out of the mix when they traded Lucas Giolito and Reynaldo Lopez to the White Sox earlier this month for outfielder Adam Eaton.

Giolito and Lopez figured to fit prominently into the Nationals' 2017 plans, perhaps the first two starting pitchers summoned by the organization to take the mound should something happen to one member of the original quintet. But their departure to Chicago via trade leaves this team with a significant dearth of quality rotation depth.

For now, the choice would probably be A.J. Cole, who made eight big league starts this season and impressed at times. (Though it should be pointed out the right-hander still finished with a 5.17 ERA over 38 1/3 innings.)

Cole, suffice it to say, still has work to do to become a reliable major league pitcher.

Austin Voth, another right-handed prospect in the Nationals farm system, would be among the leading candidates as well. The 24-year-old righty spent the entire season at Triple-A Syracuse and more than held his own, going 7-9 with a 3.15 ERA in 157 innings.

After that? Well, the pickings are awfully slim.

The Nationals did sign Jacob Turner, a one-time top prospect who has bounced from organization to organization over the last three years, to a minor league contract. But Turner, owner of a 12-27 record and 5.09 ERA in 79 career major league games (53 of those starts), is hardly a sure thing.

You may wonder if the Nationals really need to worry about pitching depth beyond that. The answer is a resounding yes.

No matter how talented a five-man opening day rotation may be, teams simply aren't able to keep their rotations intact through a full season. The Nationals this year needed 20 starts from five different pitchers who weren't part of the opening day rotation (Cole, Giolito, Lopez, Yusmeiro Petit, Mat Latos).

They needed 27 starts from four pitchers who weren't part of the opening day rotation in 2015, 13 starts from three pitchers in 2014 and 25 starts from five pitchers in 2013.

Pitching depth, as it turns out, really is significant.

The Nationals have plenty of time to address this need before the start of spring training, and general manager Mike Rizzo may be able to sign somebody with big league experience who is willing to take a minor league deal with the potential to crack the major league roster with a strong spring.

That's what the Nats hoped Bronson Arroyo would do for them this season, but the 39-year-old right-hander never got healthy enough to build his arm up to an acceptable level to pitch in the majors again.

Is there another Arroyo out there right now for the Nationals to pursue? They could sure use one.

This time, they also could sure use one capable of staying healthy and stepping into the big league rotation when he inevitably is needed.