While it may be difficult for Leafs fans to let go of Knies, the idea of adding Pettersson is very tempting. Pettersson has already proven himself in the NHL and is known for performing well under pressure.
However, the problem with this trade proposal is that the Maple Leafs are already over the salary cap set by the NHL. In order to comply with the cap rules, they would have to make significant changes to their roster before the season begins.
Adding Pettersson, who has a high salary of over $7 million, would worsen their cap situation and likely require them to say goodbye to one of their other high-paid players. It seems unlikely that the Leafs would go through the trouble of making such a trade only to replace one star player with another.