Partey was the main man last season, with the £105m Rice, he wont be the man again, he's got off field issues and he's injury prone... to me I think the one that shouldn't have left here is Xhaka and not Partey.
Partey was more of the important player last season than Xhaka. Parteys effects in games was very visible and he was very impoortant for Arsenal. He is injury prone, but when he is fit, he is a midfield master. If he is remaining in Arsenal for the next season, It means that Arsenal will have one of the best midfield in Europe and will have plenty options in midfield for every different team they play against.
It was really surprising even though previously Sadio Mane was rumored to Al-Ahli and it seems he refused to play with his former Liverpool team-mate, Roberto Firmino. But I think it's quite the right decision so as to make the squad depth for each team in the Saudi league competitive. On the other hand, at this time it can be said that Al-Nassr has not found the right left wing and Sadio Mane can provide his services through that position. With that age, it can be said that as a footballer, Sadio Mane has won everything and he doesn't need to chase further ambitions too much. Apart from that, now the decision rests with Sadio Mane, whether he wants to join other European players in the Saudi league or not.
Move to Saudi Mane, Go make some money and play with one of the greatest footballers ever Ronaldo. A move to Saudi will increase his pockets, at this stage in his career, that is what he needs.