The Blues dominated the match until the death when substitute Adam Le Fondre hit a late brace for the struggling Royals to salvage a point.
"It's difficult to explain because we had a lot of chances," he told Sky Sports News. "We could have killed the game.
"We conceded two late goals. Especially the last one, a free kick from the middle of the park. It's difficult to explain.
"They have two players at the back post and we didn't have anyone around. It was a big mistake and we paid for that."
The result leaves Chelsea in third place in the Premier League, six points clear of second-placed Manchester City.