Over at Grantland Sebastian Pruiti does the math and concludes:

After two slow starts, Oklahoma City coach Scott Brooks needs to change his lineup, taking out Perkins and putting in Harden. I don't advocate for letting the opposition dictate your lineup, but when staying big puts the team at a disadvantage, Brooks needs to make a change.

There's some nice video illustration of his point, which I appreciate because I have only the most rudimentary understanding of basketball strategy.