Byrd plays a competent CF (yeah I know he is to blame for 2 runs last night) so unless McDonald has a spectacular bat (he doesn't) I see no reason to give McD more playing time.
Since last September 1st, Byrd is 31 for 186 with 3 2B, 2 HR's and 11 BB for an approximate slash line of .167 / .213 / 215
That's worse than many pitchers. For reference, Josh Beckett's career batting line is .145 / .181 / .226
Lin would be a better defender, and as not ready for the major leagues as he is as a hitter, he'd hit better than that.