I've argued before that Lehkonen makes a lot of sense for the Rangers. It was basically:
-The Rangers are leaning heavily on goaltending at 5v5. That's not something you bet on in the playoffs.
-Lehkonen is relatively inexpensive, has team control for another year and isn't likely to get a meaningful raise.
-Lehkonen has some of the best defensive impacts of any forward in the NHL and has a better 5v5 xGF% percentage than any Rangers player.
-He's doing that against the other team's top players and with low quality line-mates. He also has experience playing effective shutdown minutes against top competition in the playoffs.
-While Lehkonen isn't a PP guy or big point producer, he helps create a lot and only Strome and Panarin are producing at a higher rate 5v5 this season.
The Rangers could probably use a player that can help their 5v5 play a lot (even if he can't finish), especially since they're loaded with offensive talent.
Don't know which Habs fan has said he's an average 3rd liner when it comes to production, but that in general isn't a healpful concept because there is no "average third liner". Outside of superstars, you want guys that can plug as many holes as possible. There's also a segment of the Habs fanbase that straight up hates 5v5 + PK players. Same thing happened with Danault.