The decision was presumably down to McDonald and Lillibridge to take the hit, with Lillibridge likely surviving the cut due to his ability to play in the infield as well as the outfield.
While it's been a fun ride with Darnell McDonald for the Red Sox (or should that be the other way around), the likable outfielder simply didn't have the numbers this year to justify keeping him on the roster. Even against lefties, typically his specialty, McDonald has only been able to hit .214/.327/.357. Now, with a 6-man rotation and a "versatile" substitute, there was simply no room left.
By designating McDonald for assignment, the Sox have also cleared a 40-man space, which should come in handy as they start to bring players back from the 60-day disabled list.
We'll see if McDonald is able to clear waivers, but it seems unlikely given the way he's played over the last couple of years and the number of teams playing some truly awful outfielders.