I read an post from iYankees blog regard of decreasing ratio of grounders from Wang. He basically points out the trend of decreasing ratio of grounders, increasing ratio of walks and strikeouts. He doesn't know whether we should concern about Wang or not.
Make my comment short. Basically, I won't worry about Wang. Any pitcher needs to master the pitch first and then think about throwing the pitch regularly to the major league hitters. Major league hitters basically can hammer any mistake pitcher throws. Especially, Result is everything for any player who wants to stay in New York Yankees. There is no room for errors.
Wang learned sinker in 2004 and master the sinker until 2006. Before 2004 season, Wang used slider, changeup,splitters and four-seamer fastball a lot. During three year span, he basically hardly use them. Since 2007, Wang start trying to get them back which takes time and can't do it overnight.
Of course, Not only his GB% will be down but also his walks will be up when Wang starts using other pitches. However, Don't forget Wang not only get more strikeout but also give up fewer homeruns. In 2008, We certainly get the feeling of the command of other pitches from Wang get better. When Wang can master other pitches, his walks will be back to normal. He may become a pitcher who can get a groundball or strikeout at any time.
I left my comment on his blog. If you guys are interested in more details, please use the link to check them out.