By Chris Cwik

Josh Donaldson has become 2013's most unlikely MVP candidate. Entering the season, the 27-year-old had received just 328 plate appearances in the majors. Given his age, Donaldson should have already been a major-league regular, not fighting for a spot on a 25-man roster. Injuries in the Athletics infield gave Donaldson an opportunity to prove he belonged in the majors, and he did not disappoint. The rise from 25th man to MVP candidate, at that age, has rarely been seen before. While he deserves plenty of credit for his breakout this season, it's tough to say what he'll do for an encore.

Donaldson's breakout was nearly impossible to predict. The third baseman has hit .306/.388/.511 with 24 home runs in 643 plate appearances. He's also been strong defensively, according to the metrics; Donaldson's defense is the main reason he's matched Tigers slugger Miguel Cabrera in wins above replacement (WAR). Both players have produced 7.7 WAR, trailing only Mike Trout (10.1) in the American League. While WAR is not definitive, it provides a solid framework, and justifies Donaldson's inclusion in the MVP discussion.

Despite his strong season, Donaldson remains somewhat of an unknown due to his past struggles. Prior to 2013, he hit just .232/.280/.386 in 328 plate appearances in the majors. His weighted on-base average (wOBA), an advanced stat that measures overall offensive performance, sat at just .291, which is considered awful for a major-league hitter. Donaldson's wOBA during this breakout year jumped to an excellent .390. That figure ranks Donaldson as the 12th-best hitter in baseball. The huge jump is extremely rare, particularly from hitters at Donaldson's age.

Since 1969, there have been 227 players who entered their age-27 season in a similar situation to Donaldon based on playing time and wOBA. Of that group, just 23 players received at least 400 plate appearances the following year. Most players in Donaldson's position find themselves relegated to part-time roles, or completely out of the majors. Of that group of 23 who did get major playing time, only two other players come close to matching Donaldson's breakout numbers:

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*wOBA1 represents a player's wOBA pre-breakout. wOBA2 is their wOBA during their age-27 season.

Mark Bellhorn and Gorman Thomas emerge as the two best offensive comps for Donaldson. All three players produced poor numbers in limited playing time the first few seasons of their careers before breaking out at age 27.

Though the sample is obviously very small, Bellhorn and Thomas can provide some insight into whether Donaldson can continue to produce at this level going forward. Bellhorn immediately declined back to his pre-breakout wOBA, and put up just one more season with above-average offensive numbers. Thomas, however, was able to replicate this breakout, and put up similar numbers over the next four seasons. Those mixed results don't make it easy to predict Donaldson's path, but there's reason to believe he's more Thomas than Bellhorn.

Where Donaldson sets himself apart is strikeouts. Both Bellhorn and Thomas had much higher strikeout rates during their breakout seasons, and over the course of their careers. That was eventually Bellhorn's undoing, and limited Thomas' average his entire career. Strikeouts haven't been as big of an issue for Donaldson, meaning he won't be prone to low averages or have to rely on high power numbers going forward. He has a more well-rounded approach at the plate than either of those two.

The other area where Donaldson sets himself apart is defense. Donaldson may have been drafted as a catcher, but he's excelled as a third baseman. According to ultimate zone rating (UZR), Donaldson has been the fifth-best defensive third baseman in the league. Defensive metrics need to be taken with a grain of salt in just a one-year sample; even their staunchest supporters say that you need multiple seasons of data in order to make a reliable determination about a player. The fact that Donaldson had hardly been used as a third baseman since he was drafted adds some doubt to those numbers. For those reasons, it's fair to question Donaldson's ability to be an above-average fielder at third moving forward.

At the same time, Donaldson has a history at third. He played the position in college, and received high praise for his athleticism when he was entering the draft. Here's what Donaldson's 2007 draft report on MiLB.com said about his defense: "Donaldson is new to full-time catching, but has good hands and caught fairly well. Some think he's athletic enough to handle a Biggio-like move to second." While the Cubs decided to try and use Donaldson behind the plate, there was some evidence he could handle an infield spot in the majors.

There isn't much evidence of catchers moving over to third and playing strong defense. Of the 17 who produced a large enough sample at the position, eleven rated poorly according to UZR. The only former catcher who emerged as a standout third baseman was Brandon Inge, who received similar praise as a defensive infielder when he was drafted, and was listed as a SS/RHP in the 1998 draft. Inge's ability to make the successful transition to third provides some hope that Donaldson's strong defensive numbers are not an aberration.

What Donaldson has been able to do has not been replicated in recent history. That makes it incredibly hard to project whether he can retain his breakout. In the limited samples available, there are reasons to be cautiously optimistic about Donaldson both offensively and defensively.

Based on what he's already been able to accomplish, it would be foolish to count him out again.

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Chris Cwik writes for various baseball sites on the internet, CBSSports.com and FanGraphs.com. He has also contributed to ESPN and the Hardball Times Baseball Annual. Follow him on Twitter at @Chris_Cwik.