Rick Porcello, Dontrelle Willis lead Tigers under- and over-performers in April

At the end of the first month, we've got a bit more information about the team than we had at the end of spring training. After all, the games are real now. The pitchers are trying to win; the hitters are trying to win; the managers are trying to win. But how much can we really read into it?

These things we know: Dontrelle Willis has the best ERA in the rotation; Rick Porcello has the worst. Austin Jackson has one of the hottest starts a rookie could ask for, lacking but for power. Alex Avila could hardly buy a hit.

During a 162-game season, a lot is subject to change. To get a feel of who we could expect to take a step back and who we think will show improvement, the easiest way is to glance at a few stats that sum up the story nice and easy. They won't tell all the story, but they'll tell part of it.

If you want reason to worry a bit, the sheer number of over-performing players on the team right now is amazing. As always, remember these are long-run predictions. Nothing in them says a player must immediately turn things around or fall off a cliff.

Today we'll look at the pitchers. Tomorrow, the batters.

First, a quick note about the stats. I use xFIPP rather than FIP because it is a slightly better predictor of what you can expect happen. It's available at Fangraphs.com. I also included Baseball Prospectus' new pitching predictor, SIERA. The reason I look at these numbers is that they use stats a pitcher can control -- walks, strikeouts, etc -- rather rather than just tell you the results. Both numbers are scaled to ERA so you can easily see how much better or worse a pitcher is doing and both are better predictors than ERA. These numbers are for entering play Sunday.

Pitchers you could expect to take a step forward

As I've been proclaiming, Rick Porcello has had a combination of bad location and bad luck. The Tigers went to the video and found his arm slot was ever-so-slightly off and feel like they've fixed that. However it's easy to see he was getting nickled-and-dimed too. A glance at Fangraphs.com will show you his line drive percentage is better, his strikeouts and walks are better, his contact percentage is the same and yet his results are worse.

Jeremy Bonderman is another guy who is pitching worse than his peripherals. He's having problems getting out of innings, as you can see. He's also getting a bit unlucky. The same could be said for Justin Verlander.

Fangraphs recently took a look at which MLB pitchers were having the biggest under- or over-performances this season. Both Porcello and Bonderman were on the Top 10 list.

Finally, you can see reliever Brad Thomas is not having the best of luck either. Even adjusted his pitching isn't really that good. But it's not as bad as it's been.

Pitchers you could expect to take a step backward

Dontrelle Willis is the only starter in this bunch. He's stranding runners at a pretty high pace compared to his career norms. He isn't striking out a lot, and he's walking fewer batters. But he's still giving too many free bases and you could expect that to catch up to him in the long run.

The rest are relievers, and due to the limited number of innings they throw during any single month of the season you probably shouldn't take their xFIP and SIERA as gospel. However, given Jose Valverde's career norms -- his 2010 ground ball rate is twice his norm, his BABIP is half the norm, his left-on-base percentage is way better -- I feel comfortable in guessing he's going to take a step back.