I'm not sure Tesi is a huge loss right now, but he could be.
I think he is a smarter fullback than Isaako, but right now he looks like he is lacking power. He looks like he should have some power through the hips, but he can't seem to break a tackle.
If he fills out a bit I think he could become decent.
His ball playing isn't elite, but he does enough with the ball in hand to be able to finish off sweep plays, and he puts in grubber kicks quite often as well. If his tackle busting improves then his running game will become a threat to go with it.
If he can't improve his power then I think he probably just becomes an average fullback as his running game doesn't look as threatening as it should be. The big bust he had was against a prop off the scrum, but I recall early in the year that he was getting some decent early ball out in the centers and just couldn't break the first tackle.
Right now Isaako's tackle busting and therefore his running game is much better, but he can't get his head right to be consistently focussed on the game.
He showed early in the year that he can be an NRL fullback and offers something back there, but the dragons debacle happens after he gets wind that the broncos were after Hynes and it's clear that his head is elsewhere and thinking about where he's going to play next year.
What has happened since... We miss out on Hynes, Cobbo isn't ready, there aren't any fullbacks on the market and Niu could be out the door... Isaako is all of a sudden back in the frame to be fullback next year, but he's been dropped back to the wing because he dropped his bundle after the club had the nerve to look at an elite fullback option... who could've played 6 if Isaako made it impossible to drop him from the spine.