Masahiro Tanaka's velocity has dipped a bit this spring, and the New York Times' David Waldstein was probably right to react by simply saying, "Hmmm." A guy with a tear in his elbow not throwing as hard? Seems like a pretty big deal, but it is important to put those 88 mph radar gun readings in context.
Tanaka has already said he is trying to implement his sinker more in favor of his four-seam fastball, which was completely crushed in 2014. According to Brooks Baseball, his sinker was, on average, a couple ticks slower than his fastball, the latter being his primary pitch to start at bats. He also used to operate on cruise control, keeping his fastball in the low 90's and dialing it up to up to 96 mph late in games. Presumably, he will do the same thing with his sinker, which translates to keeping it around the high 80's and dialing it up to 93 or so in a bind.
Speaking of sinkers and fastballs, the change definitely makes sense. Hitters tattooed his four-seamer in 2014, putting up a .337 BA and a .633 SLG in 98 at bats against his traditional heater, which despite Tanaka's excellent command, doesn't really move that much. Hitters did pretty well against his sinker as well, with a .314 average and a .495 slugging percentage. Not great, but better than an ISO approaching .300. Batters' .302 BABIP against Tanaka's sinker also figures to decrease in 2015 with the acquisition of Didi Gregorius and a (hopefully) healthy infield.
Perhaps the biggest issue going forward for Tanaka is his pitches becoming too predictable. Without the four-seam fastball, all of his pitches are designed to be thrown down in the zone. His sinker breaks in the same direction as his deadly splitter and will only be a few miles per hour faster. To keep hitters guessing, he should probably avoid completely phasing the four-seamer out of his game, mixing the occasional high heat in there to keep hitters honest. Fortunately, Tanaka has never been a slave to the "use fastballs to set up the breaking ball" mentality. In 2014, he used his splitter, slider, or curve for the first pitch of 48% of at-bats against lefties and 45% of at-bats against righties. Brian McCann has always been known for his superb game-calling ability, something that should still be there.
The only reason why anyone should worry about Tanaka in 2015 is his command. He had a 6.71 K/BB ratio in 2014, despite only throwing 40.6% of his pitches in the strike zone, according to Fangraphs. In order to coax hitters into swinging at pitches outside the zone 38.6% of the time, his command must truly have been on another level. Lots of pitchers struggle with locating pitches immediately after elbow injuries. If Tanaka doesn't have that same deadly precision he did last season, he might have to pound the strike zone more. At that point, his velocity would be a real problem. Either way, this is probably how Tanaka is looking at his doubters as he prepares for Opening Day: