With just three assists in 18 games, Brandon Prust is actually off to his slowest start offensively to date in a Canadiens jersey. By the 18 game mark his first two years, Prust already had two goals both times, with four and five points respectively. It's not a huge difference, and in all likelihood Prust is going to start popping in a couple goals soon considering he's playing with talented linemates, but production isn't what we should be focusing on with Prust.
Last season Prust was a shell of himself. He wasn't very effective due to several nagging injuries, the worst of which being either his shoulder or ribs, whatever was preventing him from putting any weight on his shots or passes, leading to broken plays and wasted opportunities. It was frustrating to watch, but had to be several magnitudes more frustrating for Prust.
This year though, Prust has been pretty excellent. His 47% possession rate (Corsi) at even strength doesn't stick out as amazing, but Prust has played most of his minutes on the fourth line with brutal deployment, and overall has started just 35.6% of his shifts in the offensive zone. Putting up close to even possession for a player of Prust's calibre in those minutes is solid.
Prust has also been very solid while playing on the so-called third line with Lars Eller and Jiri Sekac, with a 57.8% Corsi in limited minutes. Considering those three still aren't getting a huge offensive zone push, and they've been playing the opponent's top lines, that's astonishing.
With all due respect to Prust, while the third line is very good with him on it, he's not the play driver on the line. Both Eller and Sekac have been very good without Prust, and could likely benefit from a player who can bring some of what Prust brings, but shoots more (looking at you, Sven Andrighetto). The Canadiens have a few intriguing options to fill that spot, Michael Bournival among them.
What the Canadiens don't have though, is another Prust to use on the fourth line, and without him, the fourth line has been torn asunder.
I didn't include Dale Weise here for a pretty simple reason; when he hasn't played with Prust this year, he's mostly played with Max Pacioretty, not on the 4th line. That doesn't exactly tell us anything about the fourth line.
With Prust on the fourth line, they can play extremely difficult zone starts, and while they give up a wide edge in play, it's enough that they can eat 10 minutes per game and not really hurt the team, and at the same time they allow Therrien to put his offensive lines in the offensive zone a huge amount of the time, increasing scoring drastically.
Without Prust, the fourth line just gets torn to shreds, no matter the opponent.
Troublingly, the Canadiens' coaching staff hasn't compensated for this drop in effectiveness with a drop in ice time for the fourth line, with Malhotra regularly taking 20 shifts per game (including shorthanded situations). As it stands, though Travis Moen is gone, neither Malhotra or Weise have the puck skills required to effectively break out of the defensive zone, which has led to constant shifts of opponents pouring on pressure. So far Carey Price has shut the door, but it can't last forever.
As illogical as it may sound, it's easier for the Habs to find a replacement for Prust on the third line right now, than it is on the fourth line.