Colton Parayko or Brett Pesce.
Both guys that fit into the age bracket of this team, and can likely be added without having to completely decimate depth. Even in 2 years when both Parayko and Rielly would be up for new deals, you can probably bring both back if Nylander is on his way out.
The $2.5m for Parayko, or $4m savings on a guy like Pesce are going to be instrumental in being able to maintain the depth required so that if somebody gets hurt, you're not completely screwed.
Even if you have to trade Nylander to do get a guy like Parayko, honestly, so be it. A winger is easier to find/replace than a defenceman the quality of Colton Parayko. Auston Matthews has been proven to score when placed on a line with Kapanen & Johnsson. You might already have an ideal winger or two for him in the form of Mikheyev, Johnsson or Robertson. Maybe you can get a Sammy Blais, Zach Sanford, or Jordan Kyrou out of St. Louis recognizing that Nylander has twice the term remaining that Parayko does. Maybe you trade Johnsson and get a guy like Rakell out of Anaheim.