Would I make this change if I was Kris Knoblauch?
Provided the rest of the team could pick up the slack physically and there was another guy who could reasonably replace Desharnais on the PK - assistant coach Mark Stuart (who runs the PK) I'd be looking in your direction - I'd make this change, at least for one game anyway. We could see how it goes and determine from there if it becomes more permanent for the rest of the playoffs.
Stecher might be smaller but he would bring more offence and defence at even strength to the team, which right now is exactly what they need. It would also help to create more offence with the bottom six forwards. as you have more of a puck mover at the bottom of the roster to feed them the puck.
It would also give Desharnais a different perspective from the pressbox, where he would have a different view of the game. In addition, it would give the video coaches and assistant coaches a chance to work with Desharnais improving his game, so that when he comes back into the lineup - whether that's after one game or longer - he's ready.
It would also put the rest of the team on alert that the coaching staff will rearrange the lineup if you're under-performing, and send the message that this is the playoffs, so your leash is much shorter than in the regular season.
Who would replace Desharnais on the PK?
Although Stecher didn't really spend a lot of time on the Oilers PK in the regular season - only averaging 13 seconds TOI per game - I found this little number from Canucks Army from his days with the Canucks, which means that he does have some PK prowess for sure.
If Stuart doesn't think it's enough to replace Desharnais, he could always double shift Ceci, who plays the same position on the first unit PK, or give the minutes to Even Bouchard, who spent on average 20 seconds a game on the PK. Although with the amount of ice time they're giving him on the PP and at evens, I wouldn't do it lest you tire him out within a couple of games. Maybe he tries a forward there, I don't know, I'm not a tactician, but he does certainly have options if Desharnais comes out of the lineup.