It's hard to say though. I take your point but Necas could also just be surrounded by players who are the leagues best in possession and he's actually better than Garland in possession. I would argue also that being -1.4 is negligible when youre up in the 60%.
I don't think corsi rel as a raw number comparison is very useful.
Necas is 6'2" fast and has great hands so I pretty sure he drives play nicely himself.
Well here's Garland with his linemate staple Dakota Joshua as per NST:
Garland has a CF and FF of 55% at ES without Joshua, Joshua on the other hand is 36% and 37% without Garland. Garland absolutely carries his line. It's also worth noting that when you only consider ice time with Hughes, normally a possession monster, Joshua's possession numbers barely improve at all if Garland isn't also on the ice.
I think corsi relative to teammates is much more useful than raw corsi compared across teams, especially when Carolina is a statistical darling year in and year out. They shoot the puck a ton, which makes their numbers look extremely good. They shoot the puck the 5th most in the league with 33.1 per game, the Canucks on the other hand shoot the puck the 26th most, at 28.5 per game. The easiest way to inflate corsi and fenwick is to shoot the puck a lot and the Canucks don't do that, the fact that Garland keeps up excellent possession stats despite the team never shooting the puck just shows how good he is, while Necas lives in a system where the mantra is to get pucks on the net but he's still below average at it.
Necas' -1.4 may not seem like much, and it probably isn't, but Garland's +6.2 rel will be sorely missed on a team that already allows a ton of CA.
Among Canucks skaters Garland was 3rd in CF% (2nd if you exclude Cole McWard who played a single really good game), and 1st in FF% this season, comparatively, Necas was 15th among Hurricanes in CF% and FF% if you exclude players who played 2 games or less. The Hurricanes also didn't have any players with a -CF% and only one with a -FF% who played more than 2 games. The Canucks had ten -CF% and eleven -FF% players when only considering over 8GP.