Can you define exactly what you need him to do? Just looked at some numbers.
Lowest shot on target % of any player who has taken a shot on target at the club, but taking the 5th most shots per 90
Average shooting distance of 26 yards
10th most completed passes
10th in completion %
8th in passing progressive distance
Just one completed pass in the box
0 completed crosses into the box
I've already conceded the shooting issue, it's a problem. However he seemed more cognizant of it yesterday, passing on some half chances from range and Arteta seems to be utilizing him more as an attacker on set pieces and he scored a goal from one yesterday. So some positive indicators there.
The passing stuff is more an issue with the system Arteta has been playing for most of the year rather than anything I would attribute to Partey's failings. Arteta has been trying to run a 4-1-4-1 with Partey the lone man in the middle in front of the backline. However we have used the fullbacks more often than not to get the ball forward (an issue, but not a Partey issue) which generally bypasses Partey to progress the ball, leaving most of his passing back towards the CBs or to the fullbacks himself. When he does get the ball with room to attack, the front 5 have done a poor job giving him options other than balls into the channel or over the top, until last night where Lacazette made it a point to drop deep and help create space to make passes between the lines.
As for the passes/crosses into the box, he's a pure CDM for Arteta and being told to stay back because usually we don't have a partner for him and when we do, it's largely been Xhaka. And I don't know too many CDMs making a bunch of crosses into the box anyway so that seems cherry-picked AF.
He is second on the team in successful tackles behind Lokonga but I can’t stress how poor Arsenal’s tackling stats are, he’s like 180th in the league in successful tackles
9th on the team in pressures but 7th in successful pressures (similar numbers to tackle stats, league wide just atrocious numbers, like 20th percentile on the midfield radar)
He’s not pressuring the ball, he’s not intercepting the ball or winning the ball, what is he doing that he’s supposed to be doing? Lokonga passes the eye test a hell of a lot better to this non Arsenal fan.
Again, the system has been dreadful for the most part which is why Arsenal's stats are so bad and I don't blame Partey for that. Before last night, Partey was being asked to man the entire midfield defensively with limited help, many times having to track 3 players as opponents bypassed the press with ease. So he hasn't been able to pressure much and he hasn't been asked to. The shift to a 4-4-2 last night gave him some help in the midfield and it's no surprise that he put in by far his best performance of the season as he wasn't asked to be everything all at once.
And the last thing is purely subjective but you're coming at me with the "eye test" so I don't even care. He provides security and leadership that we don't have a ton of as we have gotten rid of a lot of veterans over the past 12 months. When he has been healthy (which unfortunately has been far too infrequently), he has been excellent and he gives a sense of calmness on the field. When he receives the ball from Ramsdale or he has an opponent bearing down on him, I am not terrified that he's going to panic (like Xhaka, or even your apparently favored player Lokonga).
Also, since we are using team rankings for stats, Kante is 6th in tackles per 90, 6th in interceptions per 90, 8th in blocked shots per 90, and 11th in passes completed per 90 for Chelsea this season (removing Sarr from the rankings due to one appearance). So let's not pretend team rankings are the end all be all here.