I second the advice to leave a bit early and take a cab. If it's a close game and you really want to see it through, just be aware that it will be very crowded around the park once the game lets out. You'll need to walk a bit to get out of the crowds before you'll see any cabs. If you do plan to take the T and you don't leave the game early, walk back 1 stop toward BU and get on BEFORE Kenmore so you won't spend half an hour just trying to get on the train.
There should not be much traffic on a Saturday in August and I would guess it would be about a 20 minute ride but I would give it at least an hour to be on the safe side. You never know what ridiculous traffic snarl you'll run into on the T or on Storrow Drive.