Depends on how deep your benches are, how many players you can keep, who you already have, and what stats are used.
However I would not do that trade generally.
Carl Crawford is one of the best fantasy league players because of his odd combination of power and speed. He steals bases but also contributes a lot in the other categories unlike other leadoff hitters like dave roberts or juan pierre.
I believe that Matsuzaka will have a monster year. He has a great K/BB ratio which is a great predictor of success in baseball. Also while he is very polished and experienced, as most major leaguers have not seen him or his arsenal of five diffrent pitches including the gyroball, he will win a lot of games the first run through the league. The Red Sox will score a lot of runs for him, perhaps the most in baseball.
Lastings Milledge is a problem. He won't start in that crowded Mets outfield and he probably won't get too much playing time because of his attitude. The Mets front office is down on him and he did not play too well when he got the chance. I think he might be traded to the A's but he is a huge risk.
Huston Street is a great closer but cheap saves are easy to get. Last year in my very deep league with deep benches I had to rebuild by bullpen twice from scratch. There is a lot of turnover among relievers and I wouldn't prize them too highly.
Ian Stewart is in AA now and that is a big jump to the major leagues if you are hoping he will get there soon. Once he gets there he will need to displace Garrett Atkins at third. He is also a big risk because that is big jump for a player to make. A lot of players can't make that jump and hit the curveball on a regular basis.
You could have a few dominant years of Matsuzaka and Crawford for that. If you have the good players now then play them, don't risk that hoping you will get dominant players in the future.