Has to do with the way the Spurs won and the way they lost. Game 1 took a 9 point 4th quarter comeback complemented by a Thunder meltdown for the Spurs to win, and then in game 2, the Thunder seemed to have little problem (once they made a few adjustments) cutting a 22 point lead down to 6. The Spurs haven't put together one game where they dominated OKC from start-to-finish, whereas OKC did just that last night and didn't seem like they broke a sweat doing it.
Many Spurs fans fear the Thunder have figured out the Spurs seemingly unstoppable offense, which means the pace will shift to Thunders' advantage, turning the game into more of a grind. And typically in the playoffs, the team that's most top heavy and better defensively wins those type of games. I think that's why Popovich force fed Duncan on the block early in this game, to hopefully punish the Thunders' small ball lineup that terrorizes the Spurs' pick-and-roll and run the offense through Duncan, going temporarily away from Parker, which, in theory, would make Thabo a liability since he's a poor/mediocre offensive player. If Duncan on the block works, then Brooks has to recounter with big-ball, and the Spurs can recounter that move going back to the pick-and-roll.
Problem was, Duncan isn't 28 anymore. So now we fear that all you have to do is shut Parker in the pick-and-roll (which you've proven you can) to win this series. Without Duncan no longer being able to dominate on the block, the Spurs have no effective counter.