It's literally impossible to insulate a 24 minute a night defender. In fact Dumba's 46% O zone starts was the lowest of any of Nurse, Dumba, Spurgeon, Suter and Brodin. He also averaged 2 minutes of PK time a night.
At one point in his career he was deservedly sheltered. He hasn't been for awhile and his game has matured tremendously. When he makes a defensive gaff, they are usaully highly visible and head scratching. But they are much rarer than they used to be, and he'd probably play top pairing on over half the teams in the league. We just happen to have Surgeon as well.