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Just how bad is Masahiro Tanaka's home run problem?

The Yankee ace averaged less than one home run allowed per every nine innings last year. That number has ballooned substantially this season.

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Following his start on Sunday afternoon against the Blue Jays, Masahiro Tanaka is averaging 1.63 home runs per nine innings pitched. Among all pitchers who have worked at least 90 innings, Tanaka's percentage of fly balls that leave the park (18%) is the fifth highest, and only 0.6% lower than CC Sabathia's rate. It's higher than Rick Porcello's mark. It's higher than Kyle Kendrick's. It's higher than Joe Kelly's. It's higher than Kyle Lohse's.

Let's be clear. Tanaka is not a bad pitcher. Far from it. Tanaka is actually a pretty darn good pitcher. He also gave up a lot of homers last year. In fact, setting the innings minimum low enough to let him in (Tanaka didn't qualify for the ERA race last year because of his DL stint), Tanaka actually had the third-highest HR/FB ratio in the league. This wasn't a problem that plagued him in Japan (he only gave up six home runs in his legendary 2013 campaign with Rakuten), but Nippon Pro Baseball doesn't boast the type of hitters that MLB does. Remember, this is a league where Wily Mo Peña is one of the most dangerous men in a lineup. The longball is something that's found Tanaka here. It was his introduction to MLB, as a matter of fact. He still managed to be excellent last year. But 4% more of his fly balls are going out this year, and while 4% doesn't seem like a humongous jump, it feels like one.

Let's take a look at the two homers that Tanaka surrendered to the Jays on Sunday, juxtapose it with his season at a whole, and then put his season into context with the rest of the league. The first dinger came courtesy of Josh Donaldson, who has been an absolute monster this year.

This was not a missile that everyone knew was gone off the bat. Yet, at the same time, that pitch was probably going to be hit well. First, notice that Tanaka missed his spot. Brian McCann was set up a bit further outside, and the fastball ran further in. Against a left-handed hitter, that's probably a pretty good pitch. Against a right-handed powerful maestro like Donaldson, it's trouble. What's even more impressive is that Donaldson didn't pull it into the left field seats, but deposited it into the Yankee bullpen in right-center. It's a bit of a mistake pitch, yes, but to a certain extent you have to tip your cap to Donaldson there. Tanaka can't make mistakes to hitters of Donaldson's quality, but it wasn't overly egregious.

Here's the second homer, hit by Jose Bautista.

This was just bad. Tanaka again misses his spot, again with a fastball. This one moves to the middle of the plate and Bautista does mean things to it. It was a bad pitch to a great hitter, and Bautista made him pay dearly. It's no coincidence that both pitches were fastballs. The chart below from PITCH f/x database displays opposing batters' ISO scores against Tanaka's various pitches over the course of his career.

The data entries for September of last year and August of this year are not reflective of Tanaka's true talent. Both months contain only two games' worth of data each, and in the case of September, Tanaka's awful outing against the Red Sox. He had only recently come off the DL after his elbow failed him last July, and was still shaking off rust. Small sample sizes are the devil and are not to be trusted.

Yet hitters have been consistently teeing off to greater effect on Tanaka's four-seam and sinker this year, and his curveball was demolished in June and July. Those high marks aren't the result of small samples, either. Brooks Baseball says that Tanaka threw 27 curveballs in June and 39 in July, and both marks are consistent with his healthy start to 2014. He's only had two starts thus far in August and nobody has connected with a curve yet (he's thrown 7), but it will be interesting to see how hitters react to curveballs as the month progresses.

It's important to remember that Tanaka did not necessarily pitch poorly yesterday. He only gave up those two runs, and was pulled early in the name of preserving his innings total. It was the nonexistent offense that hung the Yankees out to dry on Sunday, and throughout the whole series with the Blue Jays. Only giving up two runs to the offensive juggernaut that is the Jays is a good day's work. Donaldson and Bautista are going to hurt you; it's only a matter of how and when. Tanaka's inflated HR/FB percentage is also reflective of a league-wide trend. The highest mark last year was old friend Brandon McCarthy's 16.3%, and this year it's Rubby de la Rosa's 19.2%. Offense is seeping back into the game, and playing in the AL East (and in Yankee Stadium, and Camden Yards, and Rogers Centre, and Fenway Park) isn't helping Tanaka's cause in preventing runs.

There's also the fact that Tanaka's pitching with a partially torn UCL. This isn't a cry for him to get Tommy John Surgery and be done with it. There have been pitchers (including Adam Wainwright) that have been great with the same condition, and multiple medical professionals said that rehab was the right course of action instead of surgery. It can't be easy to operate in such a fashion, of course. That could be impacting his control and ability to consistently get the ball down in the zone. However, it's quite likely that Tanaka is simply going through a rough stretch at the same time that the overall offensive profile of the league is getting stronger. Pitchers have had whole bad years, not just bad months. If this is what a bad year looks like for Tanaka, then the Yankees are quite lucky to have him.

Nicolas Stellini is a staff writer at Pinstripe Alley, where he writes about the Yankees and covers the Double-A Trenton Thunder. His national coverage can be found at Beyond the Box Score. You can follow him on Twitter at @StelliniTweets.