The reasons this is surprising are that
- He was the Pirates' regular first baseman.
- He was one of the few left-handed bats in a strongly right-handed lineup.
- He was the team's leading home run hitter.
- He wasn't all that expensive, earning $5.75 million in 2015.
- By non-tendering (as the lingo goes) him, they'll receive nothing in return, rather than trading him.
Losing the regular first baseman. Alvarez played 906.1 innings at first base last season, 61% of the team's total. Sean Rodriguez, who doesn't have the bat to be a regular first baseman, played 326.2 innings (22%), often as a late-innings defensive replacement. Michael Morse, acquired via a trade deadline deal, played 116.2 innings (8%). The rest of the innings were divided among players who likely won't be Pirates next year. Morse would appear to be in line to take over the position. He's by no means a good fielder, though he's better than Alvarez, who was one of the worst fielders in baseball in 2015. The questions with Morse are his health (he turns 34 in March and has stayed in the lineup enough to qualify for the batting championship only once, in 2011, when he was 29) and his bat (he was decent in his brief time in Pittsburgh but 2015 overall was his second bad season of the past three). One of the Pirates' top prospects, Josh Bell, was shifted from right field to first base in 2014 and hit well last year at AA Altoona and AAA Indianapolis (combined .317/.393/.446 slash line). He's not a big bopper--30 minor league home runs in over 1,600 plate appearances--but he's well-regarded; MLB.com rated the No. 34 prospect in baseball before the season. Morse may wind up holding Bell's place until the 23-year-old is called up in May or so.
Loss of a lefty bat. The Pirates' lineup at this point has just two left-handed hitters: Right fielder Gregory Polanco and switch-hitting second baseman Neil Walker, who's a much better hitter from the left side. Morse bats right, and Bell's a switch hitter. So yes, the Pirates look vulnerable against right-handed pitching. But just looking at handedness doesn't tell the whole story. Adjusted for their park, the Pirates were the third-best hitting team against right-handed pitching last year (using FanGraphs' wRC+ metric), trailing only the Giants and Dodgers and ahead of all of their National League Central opponents. Righty swingers Andrew McCutchen, Jung-Ho Kang, Starling Marte, and Francisco Cervelli were all above-average hitters against right-handed pitchers in 2015, minimizing the platoon disadvantage.
Loss of a home run bat. Alvarez hit 27 home runs in 2015. McCutchen was second with 23. Alvarez was third, behind McCutchen and Walker, in 2014, but he led the team with 36 in 2013 and trailed McCutchen by just one in 2012. Overall, from 2012 to 2015, he hit 111 home runs, the most on the team (McCutchen's second with 100) and, believe it or not, the second most in the National League during that period, trailing only Miami's Giancarlo Stanton's 125. But here's the catch: Of the 21 players with 100 or more home runs over the past four years, Alvarez has the third-worst park-adjusted OPS. Add his defense to the mix, and he doesn't bring much more to the table than his home run bat. It's interesting to compare him to the player who ranks just behind him in OPS+, Mark Trumbo, on that list:
Trumbo bats right, Alvarez left, and Trumbo's a year older. Alvarez walks more but strikes out more too. Other than that, they're pretty similar. They both have subpar on base percentages, Alvarez because of a low batting average, Trumbo because of a lack of walks. They have identical slugging percentages, largely a consequence of their almost-identical home run totals. On defense, the team employing both would be well-advised to hide their gloves.
Alvarez is now looking for work. Trumbo? Yesterday he was traded by the Mariners to the Orioles. There were two throw-ins--Baltimore got a relief pitcher who turns 26 next month who's pitched in 24 major league games with a 6.30 ERA, Seattle got a 29-year-old catcher with a .228 lifetime batting average in 148 games--but the trade was mostly Trumbo for, well, nothing. What this tells you is that while home runs are nice--great, even--if you can't do something else, like play defense or hit for an average or run or get on base a lot or hit doubles, there isn't that much demand for your services, particularly when you enter your arbitration years and start costing millions.
They at least could've traded him. They tried, according to the Post-Gazette's Bill Brink. They didn't find a market. See the last sentence of the preceding paragraph.