Sandin played mostly sheltered minutes and graded well analytically because of that. He's good at exiting the zone and is creative enough to be useful in the offensive zone. He always showed a desire to be physical but had physical limitations that were routinely exposed against teams with heavier forechecks (Bruins, Blues, Islanders, Tampa, etc) He is not crafty enough to be a good enough defender given his physical make-up. Between the two, Liljegren has always been the stronger prospect.
Also, and maybe it has nothing to do with the reasoning behind the move, but given the minor contractual hold-out in the summer, this registers as more of an authoritative move by Dubas. It rebuilds/establishes credibility among some detractors/peers/agents who feel he's caved in previous contract negotiations.