The Blues were tested by the Danish champs for much of the contest at Copenhagen's Parken Stadion before the West Londoners scored three times in 11 minutes at the end of the match to add to Mata's 33rd minute opener.
The Spanish midfielder, who finished with a brace, was impressed by the way that Nordsjaelland went about their business and suggested that a four-goal loss was a touch severe.
"After seeing the ball possession, to win 4-0 is harsh," Mata told the club's official website.
"They played well on the ball and had some quality players. It wasn't as easy as 4-0 sounds and they had a good opportunity to score at 1-0.
"Nordsjaelland have very good players in the midfield and play it short from their goalkeeper, so I liked their style and appreciate their efforts."