Coaching issue rather than a player issue.
When Babcock was coach he played Matthews and Marner on different lines, because both are capable of driving offense, and separated it makes it harder for the opposition to shut them down. That is why even when they were just 19 + 20 years old they performed well against Washington and Boston, even as underdogs and outmatched in those series.
However Keefe hasn't figured this out yet, which plays right into the hand of the opposition game plan as both CBJ and Montreal even as underdogs only need 1 good shutdown line of players to match-up against Leafs top 2 players and every team has a few good defensive players able to do this.
People can't understand why Matthews and Marner are getting shutout and shutdown by CBJ and now MON now 23 & 24 years old, but the answer is really quite simple .. "Don't put all your eggs in one basket" . Keefe thinks the solution is more ice-time as he overplays Matthews and Marner, which reduces their effectiveness particularly late in games.
This is more about poor game plan, then it is execution by the players. This is only going to become more prominent an issue/problem as the opposition gets stronger.
It also explains why Nylander, Kerfoot and Galchenyuk are outplaying AM and MM statistically, as Montreal is focused on Matthews line and not deep enough to shutdown multiple lines. So they take out the most dangerous line, and willing to take their chances against Leafs secondary scoring vs their own to decide games. Result being some close 2-1 games and now a 3-2 game series.