Presumed Plan : Werth in left. Bryce in right. While there's a small chance they offer a QO to Span, his injury situation and the cost (almost 16 mill) make it more likely that they don't. There's also a possibility they re-sign Span, but the deal would have to be cheap and short so again, more likely that they don't. That leaves Michael Taylor in center as the most likely scenario. Backing them up will be Clint Robinson and Matt denDekker probably has the inside track for the 5th OF spot.
Reasoning on Presumed Plan : Bryce plays. If I have to explain that to you I'm not sure why you are reading this.
Werth plays. Ignore the contract for a second. Werth hit well above average in 2012-2014. 2015 wasn't good but at the same time after returning from injury he hit for power and walked a fair amount. The former was a big worry as injuries can sap power pretty easily and had done it to Werth during his first comeback in 2015 and times before. So assuming he can boost that average back up he'll be a good hitter and worst case he'll be a prime Danny Espinosa, who was a useful piece. Also he's paid a ton to play.
You would prefer if Werth, who is aging, didn't have to play outfield, but 1B is unavailable and there is no DH so you work with what you have.
Span was a key figure for the Nats the past 2 seasons, hitting better than anyone expected he could in 2014 and giving the Nats solid CF defense. However this past season was almost a complete wash as hip and core issues made him miss 100 games. A qualifying offer is tempting as it would only bind the Nats to Span for a single season. However that is a lot of money to throw at a guy who couldn't get on the field for a number of reasons. Also his defense is likely slipping a bit due to age so a lot of his value is going to be maintaining that offense. He was able to do it while playing in 2015, but that's still a gamble. A FA deal would produce a number more in line with what he'd get on the FA market but it would also almost certainly have to be multiple years, which the Nats don't want to give thinking Taylor at most needs one more year. So the easiest thing to do is let Span walk.
Michael Taylor played CF for most of the year and was excellent in the field. He struggled a fair amount at the plate but that was his first full season. Expectations are that he'll improve over time and he showed a nice tendency for hits in big situations. Whether that's repeatable or not (likely not) you'd still rather see that than the alternative.
Problems with Presumed Plan : Bryce is not a problem.
When I say we'd rather have Werth not play outfield that isn't a minor concern. He's been flat out bad since 2012 and last year was bordering on terrible. Is it the jumps? The legs? I don't know. What I do know is that Werth in LF means more hits dropping in than should and a necessity that the CF be a very good fielder. The other issue with Werth is the same as has been for a while. He's injury prone playing 81, 88, and 127 games in 3 of the last four years, and he's not getting any younger. Chances are very good another couple of nicks and bumps take away 40+ games from him in 2016 and the wrong nicks and bumps could severely effect his offense which is the only reason to play him. Last year was bad, but there's legit potential for far worse.
It's quite possible that Michael Taylor will never really hit in the majors. He had that extremely high strikeout rate in the minors that serves as a red flag. It's common for guys like that to end up being AAAA players, who look good mashing minor league pitching but can't get over the hump to be consistent in the majors. And while he has nice power the guys who swing and miss like him tend to have great power. Will it develop? That's still a fair question, but Taylor isn't a young kid anymore. He'll be 25 to start next year and should be beginning to peak. The Nats can afford a question mark bat like his in the line-up if everyone is healthy, but if not he helps create a hole.
My take : Bryce plays
The rest of the OF is a puzzle that offers one reasonable solution. Werth pretty much has to play. I think that even ignoring the contract. If he didn't show power at the end of the year things might be different but he hit with pop all through August and September so the potential is still high that he can have a very good offensive season. As long as that potential is there and reasonable, you have to play him.
If you have to play Werth you need a CF who can compensate for him in the field and as of today Taylor is a better choice for that than Span. I think Span could do the job, but given the trajectory of his defensive stats, his age, and his injury status, I can't go beyond "I think". Taylor, on the other hand, young and healthy and with the experience of 2015 under his belt (and better stats) I feel strongly would be able to do it. His offense is a problem yes, but he was a real prospect in the minors and deserves a real shot. There were flashes last year and it's quite likely he just tired out at year's end. As late as the first week of September he was simply "below average" and only a three week collapse (.151 with 1 double to end year) dropped him to terrible. Plus the Nats don't have any other good CF options or really any good OF options in the minors, so it behooves them to find out sooner rather than later if Taylor is their guy. Another half-to-full season of starting would help clear that up.
If Taylor is the starter, than there is no reason to keep Span around. I find it really hard to commit either a ton of money or a couple years to Span given that he broke, then broke again, then broke again, and I think broke one more time in a 12 month time frame. I'd offer something team-favored (say 1 yr, 8-10 million with a team option for a 2nd year) and if he takes it great. Taylor can bench it for a year (or as long as everyone is healthy and hitting). If he doesn't, so be it.
Clint Robinson is a good hitter, bad outfielder but passable for most situations. The worst case would be Bryce moving to CF and Clint and Werth on the corners but you'd probably even take that if those three were all hitting. denDekker isn't the plus CF you really want as a 5th OF, but he's solid and hit ok to end the year. If it's him or if it's not it doesn't terribly matter... wait
See this is the trap we fell into last year. Lots of questions, but treating the bench the same. No. It has to be different in this year. If Span is here than Taylor and Robinson are a fine bench. They have complimentary skills and potential to do well in long stints. But if Span isn't here that puts more pressure on the bench and by going with denDekker you make your bench weaker. (Plus we haven't even considered the real possibility that Robinson will be playing first at some point for Zimm) No. The Nats either bring back Span, using the QO or getting him in a cheap deal, or they sign a guy who would be a 4th OF for their 5th OF. Byrd, De Aza, Rajai Davis. Or they do something the equivalent of getting Span, like signing Parra. That's gotta be the way.
Outside the Box Suggestion : Bryce wants to play CF. He is motivated to play CF. Oddly he doesn't seem to suffer when he's put in CF despite corner OF stats that suggest he would. So play Bryce in CF. Like name Bryce the CF and then go out and get a huge corner lefty-hitting OF bat. Take a chance on Jason Heyward, who's entering his prime and can field so Bryce can shift over near Werth. Go out and get Cargo from the selling Rockies.
Remember Bryce is likely here for only 3 more years. There needs to be a plan to be playoff competitive over that time frame. You aren't going to have that type of player fall into your lap again.