Let me preface this by saying this isn't an excuse. Not at all.
But let's not pretend he's been invisible. He's got 23 G and 47 P in 54 playoff games. It's not like he's been a no show. And 14 of those games were head-to-head against probably the best two-way C to ever play the game. He had 5G and 4A ins 6GP in the Tampa series last year, then was completely shut down by Barkov and the Panthers in R2.
Matthews has taken plenty of heat, especially in those back to back series vs. BOS. But he's the least of the Leafs' problems. That said, he needs to find that next gear more consistently. See McDrai.
He's been off since after Game 2. It's obvious he's either got massive shits or is hurt. Again, not an excuse. It's not why we are down 3-1.
Keefe has not done Matthews, or Marner for that matter, any favors over the years in the playoffs either. I go back to the Mtl series as an example. In the 3 games (Game 5,6,7) where the Leafs could have finished off that series, Keefe did nothing to get them away from the Danault/Gallagher/Evans line when he had last change for 2 of those 3 games. He seemed more concerned with maximizing their minutes than putting them in a position to succeed. That Danault line was going out there with no thought to scoring, their job was to limit the damage.
Keefe never tried to get away from the matchup. Compare that to how Cooper played his stars against Mtl in the finals that year. Danault got on the ice Kucherov line got off. He matched his best offensive players against the Suzuki line seeing that A) they were not as good at defending and B) by forcing mtls most dangerous offensive line to play defense he was killing 2 birds with one stone.
Keefe's bench management has always been awful, and it still his. Leaving Matthews line on after PP to get hemmed in the other night when they were already half gassed, playing is 4th line with 3 mins left in a game you are down by 2 in.
Playoffs are a different beast. Playing against Danaults, Bergerons, Barkovs etc is not easy in playoffs when a goal prevented is as impt as one scored. Coaches and players need to adjust.
There are a lot of reasons for Leafs lack of success
Roster make up - too top heavy. Coaching - player usage, lack of accountability on players coaches everyone. It's not just one thing. There is no magic fix but a coach management team who instills a team 1st, team results over individual results mentality would go a long way.