deHaan is far from soft. He's a skilled player and the best first-pass on the team (well, maybe Leddy) and losing Visnovsky and deHaan KILLED the Isles transition game and made it impossible for the Isles to generate any offense in the neutral zone.
It's one thing to say that Donovan, Mayfield and Strait "played well" - i.e. they didn't hurt us with crippling mistakes or turnovers (though Strait had a couple of doozies that Halak bailed us out on) - but they didn't help the offense much at all. It was Hickey's rush and pass to Frans that led to the only goal (a softie) and one of the few times the Isles gained the zone and made a play, generating a scoring chance.
There aren't many big/tough defensemen that can ALSO skate and make a pass. Those are typically called "Pronger" or "Doughty" or "Subban". The Isles desperately missed Hamonic and while they can compensate for a short time, losing BOTH Visnovsky and deHaan made the offense essentially zero. Puck possession went away completely, playing right into the hands of the Caps, to their strengths.