As important as personnel is working out exactly what we want from that midfield two and how we want them to play. Last season it worked really well, this season not so much (albeit injuries haven't helped).
We're really poor on the transition in midfield, so if we're sticking with a two we need to do some thinking on how we counter that. In possession, what do we want from the MF - penetrating runs from deep, supporting/forming the base of attacks, long-balls into the front three? Accept that most complete midfielders should be able to do all three (and more) but each of our options actually has quite different attributes and I'm not sure how we thread them together (yet).
In possession: Against high press teams, we need CM to drop deep and receive the ball under pressure, and retain possession and when able to turn and drive with the ball from a central position, opening the pitch up. Against low block sides, we need CM to move the ball wide quickly and look to shoot from the edge, or play short through balls
Out of possession: we need to delay counters, to allow our defence to regroup. So press but not commit to the challenge. Need to be good atheletes to work back against low block sides, that try to counter.
Barlasser doesn't have the athletiscism to get back, or the open play shooting to fit our system. Work of all he isn't able to handle a high press.
Howson, the athleticism is just about hanging on as his career hits the twilight years. He can receive it under pressure, and can drive forward and pick a through ball, he hasn't got a consistent shot on him, as his goal tally proves
Hackney - is excellent at dropping receiving, turning and driving, his athletiscism is decent, but his decision making out of possession is his weakness. still, he has most of what you need to be a Carrick CM
and finally the man of this thread, O'Brien. He is OK at receiving under pressure, but isn't really a receive, turn, rive guy. He doesn't get caught dwelling so much, but he doesn't turn playing against a high press into effectively a quick break counter attack like Hackney does. He doesn't really have a shot, or pick a through ball, he does well at the out of possession stuff.
O'Brien is a good backup, Barlaser should be sold, Howson should be a backup at his age, Hackney needs a new partner.