New York Islanders Mathew Barzal Headlines Untradable Players

As the New York Islanders retool and add pieces to become a contender they’ll undoubtedly have to make some trades. Who in the Isles system is untouchable?
There are a few holes in the New York Islanders roster that General Manager Lou Lamoriello needs to address before this team can become a contender.
Like at the center position. After Mathew Barzal the Islanders strength down the middle is barren with Brock Nelson, Jan Kovar, Valtteri Filppula and Casey Cizkias making up the rest of the depth chart.
Defensively the Islanders have three good blueliners with Nick Leddy and Ryan Pulock making up the top pair and Johnny Boychuk to lead off pair number two. But with Thomas Hickey, Adam Pelech, and Scott Mayfield completing the list the Islanders clearly need one more top-four defender to round things out.
And then you get to goaltending. With Robin Lehner and Thomas Greiss both coming off a terrible year there isn’t much hope that the Isles get anything but average NHL goalkeeping.
If the Islanders are going to strengthen either position as soon as possible they’ll need to make a trade.
Which gets me to thinking, who can’t be traded by the Islanders. Who’s value is so high that they should never be traded? Unless we’re talking about a trade for Connor McDavid right now. Who in the Isles system is irreplaceable? Either because of their current value or projected value?