The loss of a handful of steals a season is expected from a guy (who is now 32) who was never fast and only had double digit steals in his rookie year back in 2002. His decline as a hitter is what we are really talking about here.
His batting average in 2002 was .249. He followed that up with a .215 average in 116 games, but his HR per AB totals saw a nice spike, and people saw 40 HR potential. He realized it with 46 bombs and hit .266. His averages following that year were .247, .234, back up to .264, .236, .267 and .260. So in 2010 he posted his third best BA, despite career low walks and career high K's.
His K rate didn't steadily go up. From 2002 to 2009 his K% rate was consistently between 31% and 34%. In 2009 his K rate was 32%. In 2010 he struck out 199 times but his K rate was still just 35%, a new career high but only slightly. in 2004 and 2006 he struck out 195 and 194 times and had a 34% K rate. His BA in 2010 was .260, which was his third highest of his career. The only thing about his stats that show any type of decline was his walk total took a dip in 2010, and his .356 OBP was the lowest of any full season he played. Even then, it was just 9 points under his previous low.
Any decline up to that point would strictly be fantasy related, because his run totals after leaving Cincy dropped about 20 a year and he wasn't going to get you any steals. People were drafting him knowing this and there was no reason not to expect a .250ish average and 35ish bombs with 80 runs and 100 RBI going into 2011.
In 2011, his K rate suddenly spiked to 42.6% and as far as I can see it is the first season he actually showed any type of regression. This season his walk rate is back up, his home runs per at bat is the second best of his career, and everything else looks normal for him except that K rate is up at 41% again, which would explain why he is hitting 33 points lower than his career average. Yes, he destroys your average now, and is certainly not the same hitter he once was, but pre-2011 I just don't see the clear cut decline you say there was with him.