Edit: How has Hendrix Lapierre been? I remember him being a faller.
He's been slowly developing. Time in Hershey did him good. He won the Calder Cup MVP en route to their B2B championship in the AHL last summer. I tried to watch him live a couple of nights back. He was steady, if unspectacular. That might be the part of his game they want him working on, though. That third line has chemistry and identity issues it's still working through, but he was solid enough. The line isn't relied on to pitch in a ton of offense, and his current role is not matched to his skill set terribly well. As injuries come, I expect he'll get chances in the top six.
Fully two lines are hot right now. It's not easy to develop line chemistry with only two lines and the bench to work with, but Carbery isn't going to mess with what the Dubois and Dowd lines have cooking at present. He's leaving those intact. The coaches are shuffling the first and third line personnel around to find fits.
With the veteran fourth line playing so well, that also means the personnel on the third line rotates out of the lineup to give players playing time. Lapierre isn't so well situated as to be guaranteed of playing 82 games yet.