Pietrangelo (personal) doesn't have a timeline for his return, Ben Gotz of the Las Vegas Review-Journal reports. He's no longer listed on the Golden Knights' roster, per the NHL media site, indicating he's been placed on the non-roster list.
At this point it's probably safe to assume Pietrangelo will miss Vegas' next few games at a minimum. Look for rookie Daniil Miromanov to remain in the lineup until Pietrangelo is ready to return.