To be fair, Tanev does only have 1 in the 6.5 games since ZAR got hurt, so maybe you're on to something
Tanev had 8 points in the previous 26.5 games before ZAR got hurt, which is pretty much a 25 point pace per 82 games. That's alright for a 3rd liner, but you want to see him closer to 30 or 35 points per 82 games with the usage he has gotten. His analytics haven't been good in the last 2.5 months and ZAR's analytics away from Tanev are actually substantially better than both Tanev's analytics away from ZAR and their analytics as a duo.
I think Tanev has limited offensive tools beyond his speed, so if he's not playing with guys who can generate some offense, I think he's going to seriously struggle to produce.