Karlsson didn't need two years post draft. He played his D+1 in the SHL. Started his D+2 in the NHL, was sent down to the AHL for 12 games and then brought back up as a regular. He played 60 games in the NHL during his D+2, easily getting #1D minutes for the second half of the season.
John Carlson was taken in the same draft. Played his D+1 in the OHL, but played in the AHL in the playoffs. His 3 points in 16 playoff games are similar to Liljegren. In his D+2 he split between the AHL and NHL. Registering 39 points in 48 games in the A, and 6 points in 22 games in the NHL playing 3rd pairing minutes (some 2nd pairing). Then he was fulltime NHL for his D+3 playing first pairing minutes.
I have watched Liljegren play a lot this year -probably 55 games. From the summer showcase, to his first rookie tournament game, where he was widely and loudly criticized, but he showed me exactly what I was looking for - while having bad luck, to his pre-season with the Leafs, AHL regular season and playoffs plus the WJC. Many games I watched from the stands where I solely focused on Liljegren and documented everything he did. The tools are all there, so is the toolbox. The major flaws that some claimed he had are not. He has been kept on a tight lease offensively so he could concentrate on the defensive end - and has done great in that area. I was worried that he would let loose at the WJC, but instead he played extremely solid and didn't give away the puck in bad spots like Brannstrom did several times. He would have had a decent number of points if secondary assists were recorded the same way as they are here, but I didn't give a damn about how many points he had at the WJC.
I will be very disappointed if he doesn't follow the Carlson path - next year mostly in the AHL, but while being given more range to show his offensive talent. With some games for the Leafs, mostly on the third pairing, especially after the WJC. And then a regular during his D+3 with top pairing minutes and PP time.
Others will say this is asking too much, too soon, and I have no idea how prospect development to the big club is going to go under Dubas, but I think he has exceeded expectations at every step since the draft, and I feel that other dramatically underestimate the percentage of first round drafted D who play a decent number of NHL games during their D+2 and then are regulars during their D+3.