Said it on here a few times, I've followed Greenwood's career since he was early teens, he was prolific as a '9' for the Mackems, probably considered one of the brightest english talents, so much so Arsenal nicked from him Sunderland at 16. After a few months they moved him back to a '10', said he lacked the pace & physicality to be a '9' and for me they are absolutely correct. He didn't settle well in the smoke and there were constant issues around his 'attitude'. As a result he went to Leeds at 18 and signed a pro contract. They played him as a '10' in there U18's/U21's and he was fairly successful. Bielsa came along seen his technichal ability and work ethic and felt he would be best suited to a wide 8 in his system, tbf to Greenwood he took his opportunity and did fairly well. Problem is I believe it's left him confused, he thinks like a striker, his first thought is how can I score? So when played in more creative 8/10 roles he makes poor decisions, also when played as a wide 8 as we have done in the main, he often leaves his full back with no protection.
O'Brien - he played his best football at Huddersfield as a left sided 8 in a midfield 3 in a team who largely played counter attacking football, he's great at driving forward at pace and joining the front players. Boro don't play like that in general, so his major strengths are lost. He had his best game in a Boro shirt away at Leicester, we sat in with a back 5 and played on the counter he played slightly ahead of Howson and broke at every opportunity, suited him down to the ground.
Greenwood I think will end up at Hull, he's their kind of signing, O'Brien I'm not sure, more & more teams are playing a possession based game even in the Championship, his style doesn't real suit it and his wages will be premier league level, with still a couple of years on his contract, he might just kick his heels at Forest for a season and move one next season.