Rick Porcello is why the Red Sox won't sign James Shields

The Red Sox could use free agent starter James Shields, and let's not pretend otherwise. He ranks 11th in the majors in ERA+ over the last four years, and threw more innings than anyone in baseball in that stretch. The Red Sox don't have anyone like that in their rotation, as Clay Buchholz has that kind of upside in terms of ERA but can't stay on the mound, and pitchers like Wade Miley are unproven in the long term. There is a space for Shields in the rotation, one that would push Joe Kelly to the bullpen, but there are also reasons for Boston to avoid Shields. One such reason is Rick Porcello.

There is an opportunity cost to signing Shields. The Red Sox are over the luxury tax threshold this year, sure, but they aren't going to want to be over it in 2016, and chances are good that they only have room on the roster for one huge money pitcher. Porcello won't necessarily be that guy -- in the sense he's unlikely to command $150 million or $25 million per year or anything like that -- but he is a free agent after 2015, and something in the Shieldsian $18 million range seems possible. If the Sox sign Shields now, their chances of having the money Porcello will end up wanting in a year are slimmer. That matters.

What Rick Porcello will be in 2015 is still a bit up in the air. He has seemed mediocre for years, but part of that very well could be the fault of the Tigers' inadequate defense behind him. It's difficult to overstate how terrible an infield with Miguel Cabrera at third base and Prince Fielder at first base is for a pitcher who lives and dies on his grounders getting scooped up. Considering that, it wasn't a shock at all to see that 2014's infield with high-quality defender Ian Kinsler at second, Cabrera at first, and Fielder playing in a different uniform 1,200 miles away resulted in a 3.43 ERA and Porcello's first normal batting average on balls in play since he was in the minors.

The Red Sox have Pablo Sandoval at third, Dustin Pedroia at second, and Mike Napoli at first for at least one more season. Pedroia and Sandoval are Gold Glove caliber defenders, and Napoli is a far better first baseman than he ever was a catcher: Xander Bogaerts is still a bit of a question defensively, but one potential hole beats the hell out of the automatic ones Porcello is used to dealing with. So, it's reasonable to believe Porcello will once again produce an ERA+ somewhere in last summer's 116 range, especially when all that kept him from that in 2013 was the defense behind him. The whole issue might actually be that simple: FIP, a run estimator which removes defense from the equation, has Porcello down for 3.91, 3.53, and 3.67 in his last three years, and he's valued as a three-win pitcher in each. Porcello very well might have already proven that he's going to be a number two starter for the Sox and whoever has him after 2015.

There is also the chance that the quality of Porcello's grounders has as much to do with his lofty BABIPs as the fielders trying to wrangle them, and that 2014 is the outlier, but that's what scouting and the 2015 season are for. The Sox don't need to extend Porcello now rather than sign Shields, but by not signing Shields, they leave themselves the opportunity to retain Porcello if he's everything they're reasonably hoping he is.

Now, why is that opportunity worth more to the Red Sox than simply signing Shields? Boston isn't just playing for 2015: they have one of the best farm systems in the game, with much of their high-quality talent either on the cusp of the majors or recently graduated to it. Porcello will be 27 in 2016, while Shields will be 34: it's a safe assumption that Porcello will be the same age as Shields' now, or maybe even a year younger, when whatever deal he signs after 2015 expires. Shields has been amazing, and he's been one of the most durable pitchers in the game throughout his career, but at some point the innings could take their toll. That's not a guarantee, as 155 pitchers have logged 2,700 career innings -- the number Shields would be at if he were to average 200 innings per season over a four-year deal -- and Shields could end up making 156. He could also get hurt or lose effectiveness a year or two in, though, and that's a dangerous proposition for a fly ball pitcher with declining strikeout rates in a park and division as hitter-friendly as what Boston deals with.

Porcello has his risks, but so does Shields, and when you consider the merits of a ground ball guy in Fenway versus a fly ball pitcher, and that Porcello is more likely to have his best years in front of him than Shields, it suddenly becomes an easy decision for Boston. When you realize the Sox are highly unlikely to spend the money necessary to get Johnny Cueto, or David Price, or Jordan Zimmermann -- not when they couldn't or wouldn't spend the same or less money for their own proven product in Jon Lester -- Porcello and the idea of what he could be becomes that much more apparently attractive to the Sox.

Free agents Porcello's age, with his upside, do not hit the market often. The Red Sox are unlikely to spend much time on the high-end of the free agent market, but if they were to make an exception, it would likely be for someone like Porcello, who could sign for six years and come out the other side in line for another long-term deal. What exactly he'd cost is tough to say at this point, but it's the Red Sox: if they want to afford it, they can. And all indications are that they'd rather attempt to afford Porcello than Shields.