Not to hammer home the obvious but Phil Kessel is effective at producing on the 3rd line in Pittsburgh because he's a really, really good NHL player. Yakupov isn't. And Kessel did still get tons of PP time in Pittsburgh which is a resource I wouldn't use on Yakupov here.
Even still, that doesn't really address my larger point which is that if there's room on the team for a winger, the Leafs have young ones I'd rather see get the spot.