My guess is you will not see him or Sandin till 2020-21 to save on cap and expansion draft etc...

Yeah it was pointed out in a recent ACGM that bringing up Liljergen means having to protect him in the upcoming expansion draft if he hits a certain number of games.
Also on a local Toronto radio station bonuses for both young D were discussed and how rookie bonuses would affect next season's cap.

So... 20/21 season is a nice flexible timeline for both young D imo.