Depending on when you see him, Rubby De La Rosa can either look like a Quad-A caliber pitcher or a Cy Young candidate. If you caught him Sunday, when he dominated the Giants over eight innings, it's hard to believe this guy isn't an All-Star. Of course, that came after a nine-runs-in-five-innings meltdown against the Dodgers that made it look like he hardly belongs in the majors.

What is the issue with De La Rosa? His talent is so obvious, and it shows up in such bright flashes, but his overall ERA for the season is just 5.27, an ugly mark in any context. Well, he can't get left-handed hitters out.

When matched up with a right-handed batters this season, De La Rosa has been nearly unhittable. His strikeout rate is 28.3 percent, and opposing hitters are hitting just .213/.259/.313 against, while swinging and missing at 14.9 percent of his pitches. These are elite numbers.

When a lefty steps up to the plate… Well, let's just say this isn't the outlier this season:

Left-handed hitters are posting an absurd .321/.371/.622 mark against De La Rosa this season; he basically turns every lefty he faces into Nelson Cruz. And though that is a bit extreme, his overall career line shows the same struggles against lefties. This isn't just a small-sample size issue.

A young starter struggling with platoon issues isn't terribly surprising, because most haven't developed the ability and confidence to throw their secondary pitches that allow them to neutralize splits. Look at Carlos Rodon's issues with right-handed batters as a result of his inconsistent changeup for proof. De La Rosa doesn't really fit this bill, however, as he certainly isn't lacking in confidence in his changeup.

De La Rosa throws his changeup 23.9 percent of the time this season, according to BrooksBaseball.net, making it his second-most used pitch overall. Against lefties, naturally, he uses it even more -- 32.9 percent of the time. Unfortunately, though he does have an 18.8 percent swinging strike rate with that pitch to lefties, it hasn't been much of an out pitch overall.

Basically, when lefties do make contact with his changeup, they tend to crush it. He has allowed nine extra-base hits on 207 changeups this season, good for a .304 ISO against. His fastball is nearly as bad, but that also isn't nearly as concerning; the whole point of leaning on the changeup is it is supposed to be an out pitch, a way to mitigate the natural platoon issues that crop up.

Last season, he saw similar, though less extreme, results. Lefties hit .318 with a .177 ISO against the changeup -- both marks are worse than what they managed against his fourseamer. So, De La Rosa's changeup is a weapon against lefties, right up until the point when it isn't.

What do we make of De La Rosa, then? There are certainly reasons to be optimistic about him as a post-hype breakout candidate, even with his ugly ERA and home run issues. His 3.32 xFIP suggests that better things are ahead if he can get his home run troubles under control, but this might not just be a case of bad luck -- he's getting blasted. According to BaseballSavant.com, De La Rosa's average batted-ball velocity allowed this season is 89.6 MPH, the 34th highest mark in the league.

With a changeup that darts like his does, it's easy to dream on De La Rosa figuring things out. For my part, I am a believer in his stuff eventually winning out, but there are warning signs here to be certain. If he can improve against lefties, De La Rosa really does have a huge ceiling, given how dominant he has been against right-handed hitters. 

The tools are all there, it's just a question of whether De La Rosa can figure out how to use them.