Lilijgren and Sandin don't do anything far better than Dermott, not yet. Dermott is one of our best defensive players without the puck on his stick. Great at winning battles, and getting to a puck and moving it away from a forechecker. Also a great rush defender, especially on odd-man chances with his ability to dive and intercept passes. His offense unfortunately isn't there, despite being a strong skater and looking more aggressive
Lilijgren and Sandin both will pass Dermott most likely, but he's looked great with Rielly and should be playing 17-18 minutes a night with Rielly every night.
Rotate Sandin/Lilijgren/Holl on the bottom pair if you want, but give Dermott a shot to establish himself playing in a role he's shown promise in.
Rotating Dermott when he's finally gaining some confidence doesn't make sense. Give him a real shot and see what he's capable of accomplishing