First, the Rangers will have to change the setup of their power play. Currently, for the most part, Panarin with possession in the offensive zone works the right side of the ice. He has the option to either shoot the puck or lure defenders in and find an open lane for a cross-ice pass. Taking off Zibanejad, with his elite shot, eliminates that threat. That would mean Panarin would have to shift over to the left circle if the Rangers want the one-time option still in play. Sometimes with how the Rangers rotate on the ice, Panarin is at the left circle anyway. But if Zibanejad is taken off - rotating and finding shooting threats will be tougher.
However, with that in mind, Lafreniere was one of the top 5v5 scorers on the Rangers last season. He was ranked 43rd in the NHL for that metric alone.
Putting him on the 1st power-play unit could result in a successful power-play unit, in addition to adding him to the right side of the play as a left-handed shooter increases the threat of goal-scoring on opponents.
More importantly, putting him on the right side of the ice (as a left-handed player) opens up another shooting threat. When operating on your off-side in the offensive zone, it's easier to handle passes and can allow for quicker cross-ice passes. Plus, Lafreniere will be able to be in a shooting position if he decides to crash into a slot coming in from the right side, which is not something Panarin can really do.