I think its probably fair to say he's been a little 'under whelming' partly due to last season being pretty poor for many of the players with the World Cup in the middle of the season being responsible for a lot of injuries in 2nd half of season and poor form.
He's not been a 'standout' this season either, partly because we've got players like Son, Maddison, Van de Venn, Romero, Vicario and others having very good seasons.
But it would be wrong to say he's been poor or not contributed this season - his goal and 3 assists in 10 PL games this season makes him the 3rd highest goal contributor behind Son (8 goals, 1 assist) and Maddison (3 goals, 5 assists). I suspect some individual 'man management' and maybe finishing coaching would help him increase his numbers.
Johnson coming in, if he can replicate his link up play with Son and Maddison from the Palace game, might take some pressure off him too.