First things first, this is all on the stats. OKC are obviously not a bad defence themselves and may be prone to lacking their usual punch without Westbrook, whilst the Celtics are going to light up no-one and don't play at the fastest of paces.
The Under is a combined 10-1 (OKC 5-0, Celtics 5-1) when these teams are playing on 1 day of rest. Also, Under is 5-1 when the Celtics are a home underdog of 5-10.5 points, and 24-10 Under when OKC are favorites of 5-10.5 points. That's an emphatic combined 29-11 Under when these teams are dealing with their respective home dog/road favorite spread in this range.
The Under is also 4-1 at TD Gardnes and 6-2 in the Thunders last 8 on the road. The stats are pretty one way here, so hopefully they aren't misleading us.