Platooning Rickie Weeks

Earlier this week, Lloyd McClendon disclosed that he envisions left field as a platoon between Dustin Ackley and Rickie Weeks. Colin already tackled some of the implications of this move but I wanted to take a look at what this might look like, statistically. I completed a similar investigation after the Mariners traded for Justin Ruggiano and the methodology will be mostly the same.

From that previous article:

To determine the projected platoon split for any given player, we must regress towards the mean to estimate based on their observed performance. Once we've established an estimated platoon split for all the parties involved, we must adjust each player's performance based on the estimated percentage of plate appearances against both left- and right-handed pitchers.

I'll be using the combined Steamer and ZiPS projections found on the FanGraphs depth charts for each player's 2015 offensive projection.

I'll walk through my calculations for Rickie Weeks and then show you what we might expect from platooning him. Weeks has a career .341 wOBA; his wOBA against righties is .331 in 3426 plate appearances and his wOBA against lefties is a robust .370 in 1274 plate appearances. That gives us a difference of 11.4% in his handedness splits. Using the same methodology -- ((.114*1274)+(0.0611*2200)/(1274+2200)) -- gives us a regressed platoon skill of 8.1%. Using his combined Steamer and ZiPS projected wOBA, I "centered" his regressed platoon skill based on the percentage of plate appearances Weeks has had against lefties (27%). This gives us an estimated platoon wOBA of .295 against righties and .320 against lefties. Those two marks translate to a wRC+ of 91 and 108, respectively.

Dustin Ackley will be splitting the majority of his time in left field with Weeks so let's calculate their combined platoon skill. First, we must regress Ackley's platoon skill. Here are the relevant inputs:

Plugging these in gives us an estimated regressed platoon wOBA of .317 vs. RHP and .292 vs. LHP; a wRC+ of 106 and 88, respectively. Combining the platoon skills of Rickie Weeks and Dustin Ackley requires a simple weighted average based on the projected percentage of plate appearances against righties and lefties. This gives us a combined estimated wRC+ of 102.

What about our original platoon partner, Justin Ruggiano? What if he split time with Ackley in left field? Ruggiano's regressed platoon skill is .298 wOBA vs. RHP and .321 wOBA vs. LHP. Combining those marks with Ackley's regressed platoon skill gives us a combined estimated wRC+ of 102, marginally better than a Weeks/Ackley platoon (tenths of a percentage point).

What about Seth Smith? What do his platoon splits look like now and what might they look like if Rickie Weeks spent some time in right field (or if Seth Smith spent some time in left field)? Smith's regressed platoon skill is .331 wOBA vs. RHP and .285 wOBA vs. LHP. A platoon with Smith and either Weeks or Ruggiano would be very similar as well, a combined estimated wRC+ of 106 for both platoon mates.

Rickie Weeks might split time with one other leftie on the roster, Logan Morrison at first base. Morrison's regressed platoon skill is .329 wOBA vs. RHP and .308 wOBA vs. LHP. A Weeks/Morrison platoon has a combined estimated wRC+ of 106.

Weeks' low offensive projection for this year really hurts these calculations. Both ZiPS and Steamer are not optimistic about Weeks' production now that he's moved to Seattle. Based on the projections, platooning Dustin Ackley and Rickie Weeks will be a very marginal improvement (a few percentage point by wRC+). If Weeks is splitting any time with Seth Smith or Logan Morrison, the platoon actually hurts the overall projected production from either position.

Obviously, the Mariners are much more optimistic about Weeks' bat or else they wouldn't be positioning him to be a major contributor in 2015. The fact that Lloyd McClendon is confident enough to say he's going to play Weeks a third of the time in left field tells me that the Mariners are expecting more than just a one percent improvement. If we assume that Weeks will approach his offensive average from the last three years (a .325 wOBA), the left field platoon will be a much more significant improvement over just throwing Dustin Ackley out there every day. If the platoon Rickie Weeks experiment doesn't work out, the flexibility that's been built into this roster means the team shouldn't been any worse off. That's a bet the Mariners are willing to take.