The Blues were 2-1 up at St James’ Park after goals from Mata and Frank Lampard cancelled out Jonas Gutierrez’s first-half header.
But Moussa Sissoko scored twice on his home debut to condemn Rafael Benitez’s men to their second defeat in six Premier League outings.
Saturday marked the third time in recent weeks that Chelsea have been pegged back, after squandering 2-0 leads against both Reading and Southampton.
And Spain international Mata admits that the west Londoners must eradicate the slip ups as they bid to secure Champions League football for next season.
“We did what was most difficult: a comeback after their early goal, but later on we didn’t manage the score and in the end we lost 3-2, so we went back to London with no points,” Mata wrote on his blog.
“This is an aspect we worry about and something we must do better: in the last few games we were leading the score but we ended up with a draw or a defeat.
“We have to keep working and try to get as far as possible in each of the competitions.”
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