On top of what had been an excellent season for Bumgarner, he tossed a Major League record 52.2 postseason innings. Overall, he threw 270 innings over the course of 2014, excluding his Spring Training starts. Bumgarner could have easily thrown 300 innings in 2014, combining Spring Training, the regular season, and the postseason.
That's a lot of innings.
Bumgarner's fastball has started a tick slower to begin 2015 than it was at the end of 2014. In October of last year, Bumgarner was pumping 93.65 mph. Now, he's throwing just 92.25.
That's 1.40 mph lost on his heater. While it doesn't seem like much, it does mean a lot in a big league setting, plus it could be an indication that Bumgarner isn't where he was and has overworked his arm.
Since 2000, five pitchers--Bumgarner, Curt Schilling, Josh Beckett, Randy Johnson, and Cliff Lee--have thrown over 38 innings in a single postseason. None of those five pitchers showed a serious sense of fatigue, at least from their ERA, the following season.
Since Pitch F/X data wasn't implemented by Major League Baseball until 2006, of the pitchers listed above, Cliff Lee's 2009 postseason is on record. So, therefore, I looked at his velocity before and after his 40.1 inning 2009 playoffs.
In 2009, Cliff Lee worked an average 91.50 mph fastball. In 2010, his fastball speed actually took a slight increase to 91.90 mph. There was no serious sign of fatigue, as Lee went on to go 12-9 with a 3.18 ERA and 10.28 strikeout-to-walk ratio that year.
Bumgarner's season hasn't been pretty this year, as he has gone 1-1 with a 5.29 ERA and a 5.50 strikeout-to-walk ratio in 17 innings. His FIP, however, suggests that he has been subject to some bad luck thus far. His FIP is over a run lower than his ERA, at 4.09.
Bumgarner may have started the season off on the wrong foot, but there is absolutely no reason to be worried about his status longterm. Everything will be just fine.