I think there is a good defender in there. He is quite quick, good in the air and a keen competitor. He is still very raw though and probably doesn't read the game as well as he might. I think he also overplays, I imagine the message given to him by Koeman will be get rid of it and if he starts doing that he will be more effective.
The game that stands out for him is Spurs at Home last season. for 45 minutes he was all over the place. I don't know if someone said to him at half time to change what he was doing or he worked it out himself but second half he came out and had a physical battle with Kane and started volleying him which nullified him. He also seemed to be quite at home doing that. He's one of those that is best when playing within his limits.
As with most of the players the issue for Mori will be patience of Koeman. I don't think he wants people who will be good centre halves in 12 months but people who are ready to play now. He doesn't seem to have much develop time leeway. He will either do as Koeman wants or not play. There are enough raw ingredients there for him to be a very good centre half but it depends on how quickly he can learn.
As indicated above, for the next run of fixtures there aren't many gimme games. Only really Hull and Swansea, the rest will be challenging. I wouldn't mind seeing us have a run with 3 at the back. He does carry the ball out of defence better and alongside Jagielka and Williams I think will stabilise him. He is quicker and better in the air than them both and a bit more aggressive, when dealing with lads like Zlatan, Costa, Giroud so he could be an effective weapon, and allow Jags to focus on blocks and Williams to organise. We'd also pose greater threat of set plays which will be a good option for us against the harder teams.