Actually, I don't think it is lack of intensity. it is likely they are playing tighter in close games than they were earlier in the year now that more is at stake.
True, they are not a playoff team this year, but they are playing like one even in this rough stretch. 2 point losses to Nuggets, Warriors, Bucks, Celtics.
The Celtics went to the Eastern finals last year without Kyrie. They are a legitimate contender. Keep in mind, the Kings are also down a main cog. No shame in losing to the Celtics, but it doesn't lessen the disappointment.