The Brazilian revealed that being beaten in Ukraine has come as a shock after a fine run of Premier League form.
He told Sky Sports News: "It was a difficult game. Shakhtar played well and deserved it.
"I don't know what happened. The Shakhtar players are intelligent and didn't give us any opportunities. Only in the last 15 minutes did we have good chances.
"We didn't play well, it is a shock following some good performances in the Champions League. Congratulations to Shaktar. Sometimes you can't play well, tonight was that."
A 2-1 defeat leaves Chelsea in second place in Group E, two points behind Shakhtar but still a point clear of Serie A champions Juventus.