Take the Mass Pike to the Copley Square exit and then go to Hynes Convention center off Boylston St. and Park in the Hynes Garage
Pilgrim Parking - Hynes Auditorium Garage - Boston | nextstop.com
Its $15 for the game - Then you can walk from Hynes the back way to Fenway (about a 10-15 min walk from the ballpark)
or take the Cahhh to Riverside and take the "T" in..
make sure you purchase your return ticket ahead of time so you wont get stuck at the crowds at the station ---and can hop right on
----another option would be at the end of the game walk over to Hynes convention center stop and get on there, wait for a "D" train for the Riverside Station so you can get a seat. You'll prolly have better chance of getting on with an empty train and have less wait than at Kenmore square..-
and you might even get a seat.