arthur's average time on the pitch is 30 minutes. of course his whoscored score will be shit: the more successful passes, tackles, interceptions, key passes (etc) you make, the higher your score will be. you won't do shit in 20-30 minutes. he had a good cameo vs roma; whoscored rated him 6.37... and whoscored also ignored the coppa and supercoppa games.
anyway, arthur's stats aren't that bad if you per 90 min metrics (cl first, then league):
of course he's not our best mid, both loca and mckennie are having a better season, but whoscored makes bench players worse than they actually are, while per 90 metrics do the opposite: as a sub, you're usually playing vs a more exhausted side, and you couldn't possibly keep your performances at the same level for full games.