(Photo by Mitchell Layton/Getty Images)

Washington Nationals outfielder Bryce Harper will begin his minor league rehab tonight at Class A Potomac, getting back on the field for a game for the first time since April 26, when he tore a ligament in his thumb sliding into third base.

Harper will hit second and play left field for three innings, the first of between seven and nine rehab games. The most interesting development will come tomorrow: The Nationals plan for Harper to play center field – Harper’s favored outfield position – for five innings.

In Milwaukee, Manager Matt Williams reiterated Harper will prepare to play all three outfield spots when he returns. Harper playing center field so early in his rehab assignment cements the Nationals’ plan for Harper to play some at that position and therefore take at least some playing time away from Denard Span.

With Harper back, Williams will have many lineup iterations to choose from. If Harper plays left field, Ryan Zimmerman – who took grounders at third base Monday afternoon in Milwaukee – can move back to third base, Anthony Rendon can shift from third to second base and Danny Espinosa can sit. If Harper plays center field, Span would sit, Zimmerman would remain in left and Espinosa would play second.

When Span or Espinosa start on the bench, the Nationals will lose either their best defensive outfielder or infielder. Some advanced statistics disparage Span’s defense. Per FanGraphs.com, his UZR – a formula that takes into account arm strength, range and errors – has cost the Nationals 2.4 runs compared to an average center fielder, which ranks 12th out of 19 qualified big league center fielders.

The Nationals’ own evaluation is more in line with rival scouts’ and teammates’ perception, which is that Span is an elite defensive outfielder. According to the Nationals’ internal defensive metrics, Span is one of the 15 best defensive outfielders in the majors, qualified, corner outfielder, center fielder or otherwise.

Why the disparity? First, the UZR stat becomes unreliable when it’s used in a sample of roughly half a season. Second, the Nationals have more information available, and they are able to better calibrate data as it relates to the dimensions of Nationals Park. If you watch Span play center field and you think he’s great, it’s okay to trust your eyes.

What that means for the Nationals is, on days Harper plays center they’ll add offense but lose an excellent glove in center field. Harper played center field well in his rookie season, ranking among the league’s best in both UZR and the Nationals’ internal calculations.

They’ll lose something in their outfield defense if Span doesn’t start. But in that scenario, they would maintain their best infield defense, because Espinosa could stay at second and Rendon could play third while Zimmerman stays in left.

Once Harper returns on or around July 1, every move Williams makes with his lineup as he mixes and matches on a game-by-game basis will cause several ripple effects. For tonight and the next week, Harper will simply try to regain his footing in the outfield and his timing at the plate while testing his surgically repaired thumb.

Catcher Wilson Ramos, expected to rejoin the Nationals on Thursday in Chicago, will play nine innings tonight in Potomac. We’ll have a report from Pfitzner Stadium on both Harper and Ramos later on.