The team has two legitimate NHL goalies, seven legitimate NHL defensemen, and at least 12 legitimate NHL forwards. While there is certainly room for improvement, there are no major holes on this Leafs roster right now. Adding a league-average player won’t move the dial much at all.
On defense, the Leafs don’t need another third-pairing option. Perhaps they can find an upgrade over Justin Holl in the top four, but if you’re giving up a first-round pick, you better be sure that you’re acquiring a significant upgrade. At forward, Pierre Engvall would likely be the one who comes out of the lineup if an addition was made, but he’s grading out extremely well by Evolving Hockey’s RAPM at the moment:
Giving up future assets only makes sense if the team is acquiring a difference maker.