I work at Fenway Park part-time (during the season) and the traffic depends on what time you head in there. My full-time job is in Framingham and I usually leave around 4-415pm and I can get in there and get a spot between 4:30-5 (gates open two hours before gametime so people can go in then). In your case, I would have to agree with Joe because the worst part of driving in there is the traffic after the game since most people head in the same direction, towards Storrow to get back to I90, I93, rt1, rt3. According to the Sox schedule, the game on the 24th is at 7 and the game on the 26th is at 8, you should double check the tickets. It will definitely be worse on either day since they are playing the Yankees and all the New Yorkers that came in for the game will be heading back that way. I live in Boston, closer to Dedham so I leave in the other direction plus I leave well after the game is over anyway. The Kenmore Station is less of a walk to the park then from the Fenway Station but it would be less confusing on the way back because there are 5 variations of the green line, 3 go through Kenmore but only 1 goes through Fenway.
If you are looking to stay around Boston for a little while after the game, then driving in would be best and the after game spot is the Baseball Tavern especially during Yankee games.
If you want to drive in or if anyone in the Boston area is looking to drive in anytime, I can provide some tips and tricks to parking in the area. I'm in there for most of the games and I either park for free or pay at most $2. There was the occasional time when I would be s**t outta luck and have to pay $25 to park but I have ways around that now.