It’s fair to say the Celtics began yesterday’s game in a deficit situation. On the road against the team with the third best record in the NBA, they had some basketball reality to overcome. But they didn’t count on one of the obstacles being themselves. The Celts met the enemy yesterday, and he wore both white and green jerseys. What the Oklahoma City Thunder did in their 91-79 victory that snapped the C’s five-game winning streak was hard enough. What the Celts did to themselves made their task nigh impossible against Kevin Durant, Russell Westbrook and friends. The Celtics turned the ball over 19 times for 19 Thunder points (the hosts gave up just nine points off their giveaways). And when the home team made a home at the free throw line (27-of-33 to the C’s 14-of-20), you dribble into no-chance territory. Ah, Oklahoma City, where the win streak goes sweeping down the plain. The Thunder came in leading the NBA in points per game at 107, and the C’s held them to their third lowest total for the season. And it wasn’t good enough. “I thought for the most part we defended well, but you can’t turn the ball over against them,” said Paul Pierce, who led the Celts with 20 points. “We sent them to the free throw line way too many times. You look at those two elements alone, if we’d have done a better job in those two departments, this is a place we feel like we could have came in here and got a win.