Sometimes, offseason acquisitions work out. Sometimes they don't. Sometimes, they are spectacular failures.
I remember going to a Cardinals game with my older brother (who lived near St. Louis at the time) in 2004. Edgar Renteria was having a down year, but had a couple good years before that. I nudged my brother and said that Renteria would be a Red Sox the next year. I was right.
Unfortunately, Renteria was a very expensive disappointment. His offense was not bad actually, he hit .276/.335/.385/.721, numbers that are not as high as his career norms, but not far off either. He also hit eight home runs and drove in 70 runs. Across the board, his numbers were down, but not to the point that he had a significantly worse season than 2004.
Defensively though, Renteria was a total disaster. He was worth -2.0 dWAR and committed 30 errors. Maybe the field contributed to this showing, but he was never given a chance to prove that. After the season, despite being on the hook for $30 million and three years, Boston traded him to the Braves for Andy Marte, who was then packaged for Coco Crisp, another disappointment. Renteria of course went on to a good year in 2006, a great year in 2007, and then crashed again. He was the World Series MVP in 2010 though. Boston is still looking for a high quality everyday shortstop, although Marco Scutaro last year was decent.