Juraj Slafkovsky was drafted because he showed the potential that he possesses in spurts and management's belief in their development staff.
Something that is often tough for bigger prospects is they have to improve their game while having to adjust to their large frame. The growing pains and adjustments to longer limbs and having to learn how to use the size adds another layer of difficulty to development. Beckett Sennecke is a great example, he was left stunned when the Anaheim Ducks selected him, but his playoff performances and growth throughout the season both physically and on the ice were extremely impressive.
Slafkovsky had a tough start to his NHL career, and as we know every first round pick is put on a pedestal and if they don't hit their potential right away, they are ridiculed. This is obviously a silly way to look at a player regardless of where they are drafted. Very seldom to players step into the league and dominate right away, it's actually a less frequent occurrence.
If Slafkovsky doesn't take a big step forward, let's not overreact
Slafkovsky is only 20 years old and it isn't uncommon for a young player to struggle, then have a good year then struggle a little more. Expectations are obviously that he takes another big step in the right direction and continues to be a dominant physical presence. He protected the puck well and won many 50/50 battles in the attacking zone, which made Cole Caufield and Nick Suzuki's lives much easier.
There is every reason to believe that he does the same thing next season and consistently going forward, but if he stumbles it shouldn't be a huge surprise. Development isn't always linear, even some of the best players have down years, after having consecutive dominant seasons. Jamie Benn had two seasons where he nearly missed 90 points, and he hasn't been able to hit 80 points since.
The point is that one season to the next can be so different, even if the offseason goes well. Quinton Byfield and Alexis Lafreniere are two further examples of players who had high expectations but didn't really find their footing until their fourth season. So, needless to say, if Slafkovsky doesn't play to his potential next season, it doesn't mean he has fallen off.