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No matter how good a starting pitcher is, a mild slump can happen at any time. Even the game's most dominant and consistent starter, Clayton Kershaw, has succumbed to back-to-back sub-quality starts. Sure, it's been almost two years since it happened, but still...

Actually, Kershaw was perilously close to starting off 2015 with a pair of outings that fell short of the quality start criteria, as he met the minimum requirements of six innings and three earned runs in his season debut against the Padres, and then the Diamondbacks hammered him for six runs (five earned) over 6 1/3 innings in his second start. Three other pitchers whom I currently have ranked in my top 15 -- Stephen Strasburg, Jon Lester and Jeff Samardzija -- have a combined zero quality starts among them to date, even though each has already made two starts.

Because a brief slump can befall any pitcher, it is clearly premature to sit any of these four aces. However, it's never too soon to look for warning signs of impending trouble. Some slumps are largely the product of bad luck, while others result from issues, such as health and mechanical problems, that can have a lasting impact.

So let's take a look under the hood and see what's behind these underwhelming performances. We just might see some things we won't like, and in time, they could present us with reasons to consider a move to the bench.

Clayton Kershaw: Velocity, frequency of contact, control, ground ball tendencies...none of the indicators for these key components looks out of whack. Kershaw has allowed a .421 batting average on grounders, and given how strong his other stats look, there is no reason to think he is to blame for the excess base hits. It's doubtful you will find an owner who is so frustrated that he'll sell Kershaw at a discount, but this may be as good an opportunity as you'll have to trade for him all season long.

Stephen Strasburg: Because Strasburg has managed to get 10 strikeouts in his 10 2/3 innings, you could be tempted to think whatever has ailed him will right itself quickly. That may not be the case. Though he is still throwing in the mid-90s, he has lost some horizontal movement on his fastball, according to the data on BrooksBaseball.net. That could account for his skyrocketing contact rate, which stands at 89 percent (per FanGraphs), as opposed to last season's rate of 76 percent. This trend is something to pay attention to in his upcoming starts.

Jon Lester: The Cubs' new ace may still be playing catch-up after dealing with a dead arm in the latter part of spring training. His fastball velocity is down roughly 1 mph as compared to last April, and he doesn't appear to be fooling hitters at all. According to FanGraphs, batters have been swinging at Lester's pitches in the strike zone much more frequently (83 percent, as compared to last season's 66 percent), and they've been laying off pitches out of the zone (swinging at a 21 percent rate this year versus 33 percent last year). The 46 percent line drive rate he has yielded may not be as fluky as that extreme number seems. Still, I expect that Lester will regain his prior form in due time and won't present owners with a start/sit conundrum.

Jeff Samardzija: Despite a 6.23 ERA, there are no warning signs here. Samardzija is throwing with his typical velocity and control has not been an issue. He only got one strikeout and four swinging strikes in his season debut against the Royals, but that result was likely matchup-driven. As a team, the Royals simply excel at making contact. His subsequent start against the Twins wasn't all that bad -- four earned runs over seven innings with six Ks -- but across those two starts, it didn't help that Samardzija stranded only half of his baserunners. Because Samardzija has some inconsistency in his past, he could present a genuine buy-low opportunity. If you already own him, though, resist the urge to bench him.