As the roster currently stands, Darnell would be a good addition. He is an average or above average corner OF, has a little bit of pop in his bat and can get on base. Facing any LHP, Darnell would be a large upgrade over Sutton and Harrison. He would also be an slight upgrade over Presley.
Here is his FanGraphs: McDonald
He is also a player that has spend the last few years in the AL East and has been involved in a few playoff runs. The other guy, A.J., is the only other player on the roster that spend signifiant time in the AL East and was involved in multiple playoff runs.
Being in the AL East and on playoff contenders is not a sole reason to add a player, but there is a need in the OF. Outside of the guy we are all waiting on arriving ..... he can help.