I looked through some news from some Japanese websites. This one is from official Japanese MLB website (http://mlb.yahoo.co.jp/japanese/headlines/?id=2120500&a=9373) updated on Jan 17th about Matsui. I asked my Japanese housemate to translate for me.
Matsui is going to go to USA 10 days earlier in order to check his right knee condition with the doctor in New York who help him operate his right knee surgery. He will then go to Tampa to check his right knee condition with Yankee Trainers again. This is not a normal action which go to USA earlier for Matsui. He usually trains with his formal Japanese team and then reports in spring training.
Right now, Matsui doesn't feel pain about his right knee, but he can't run . He is not sure he can run or not when he reports to Tampa. He understands he has to fight with Johnny Damon for left outfielder job and wants to do his best. That's why he decides to go to USA earlier.