James Shields was one of the worst full time starting pitchers in MLB in 2010 (a 5.18 ERA was among the worst). In 2011, he was one of the best. So who is he? In 2010, he had a very pedestrian 1.28 PE, while in 2011 he had a great -2.11 PE. His WAR in 2011 was 6.1, while his terrible 2010 season registered a -1.3 WAR. In 2011, he had a great 12.28 TR. In 2010 his TR was 9, which frankly isn't terrible. Shields' groundball percentage was only slightly lower, and HR/FB was only slightly higher in 2010. His FIP in 2010 was 4.24, which would have been a not too bad ERA. In 2011, his FIP was actually higher than his ERA, so while his fielders made him look worse in 2010, they made him look better in 2011. The BABIP difference was also striking as well. In 2010 it was .341! In 2010 it was .258. He also gave up more infield hits in 2010. He actually struck out more and walked less in 2010. The only things that really go against the Shields was unlucky narrative in 2010 and due for a good year in 2011, was that he gave up more line drives and fly balls in 2010. Looking at his pitch selection, he threw less fastballs and more curveballs in 2011, and that can't be emphasized enough. Other than that, the metrics seem to argue that Shields is somewhere in between his terrible 2010 year and his amazing 2011 season. Expecting a WAR of around 4 next year would probably be a good expectation.
There have been many rumors that Shields will be traded. Should he be traded? Of course it matters what you are getting back. He is going to make about $7 million in 2012, and if he does put up a 4 WAR next year, that is a WASP of 1750, very very solid. However, one has to grapple with the money problems of the Rays. At about a $42 million payroll, Shields would be 1/6th of the team's salary. They would obviously need more than 4 wins over replacement for 1/6th of their salary to make the playoffs. Assuming they need 95 wins to make the playoffs, they would need over 7 WAR for every 1/6th of their payroll. So it seems to make sense for the Rays to trade him, but they better get a very good package.