In the end, it was a walk in the Parken as Chelsea scored three times in the final 11 minutes to get their title defence up and running in earnest.
Mata continued his amazing recent form with his third goal in three games but it took a superb free-kick from David Luiz to open the floodgates as Mata and Ramires sent the European champions top of their pool.
Before that, there were as many questions as answers at the Parken Stadium in Copenhagen as Chelsea almost let another lead slip two weeks after blowing a two-goal advantage to draw their opening match against Juventus.
Frank Lampard returned to the starting XI for the first time since then, captaining the side in the absence of the rested John Terry.
The 34-year-old cleverly set up Mata's opener and led the visitors well, although doubts remain over whether he could form an effective midfield shield with either Ramires or John Obi Mikel.
It was almost exactly a year since the first cracks in Lampard's relationship with former manager Andre Villas-Boas began to emerge after his game time was similarly rationed.
But, unlike 12 months ago, there was no visible sign of disaffection from the England star tonight, with Roberto Di Matteo proving a far better diplomat than his predecessor.
Lampard's recall made perfect sense against a team Chelsea were expected to beat almost regardless of selection and tactics.
The gulf between the sides was further emphasised by the fact Nordsjaelland were unable to get anywhere near selling out what was arguably the biggest match in their short history, having switched their Champions League matches to the national stadium.
Di Matteo last night described this game as Chelsea's biggest of the group and they certainly started as if they meant business, Jores Okore hitting his own post clearing Oscar's cross, with Victor Moses poised to nod home.
Fernando Torres also made a hash of beating the last defender after a lucky break and Lampard sent Mata's cross over the bar.
Sloppiness soon crept in as Nordsjaelland, whose possession game soon got going, had a couple of sighters before a desperate lunge from Ashley Cole prevented Kaspar Lorentzen turning in John's cross.
Nordsjaelland continued to make errors, one from Ivan Runje playing in Torres, who fired at the legs of Jesper Hansen.
And it was a mistake Chelsea punished ruthlessly in the 33rd minute, Torres robbing Enoch Adu and finding Lampard, who showed great vision in feeding Mata to slot home.
Nordsjaelland continued to play as if they had nothing to lose but Torres should have put them in their place a minute before the break when he shot straight at Hansen after being put clean through by Oscar.
It was a similar story after the restart when the Spaniard was beaten to another Oscar ball by the goalkeeper but the first real shot came from John, who fired just wide after sidestepping Ramires.
Moses had the ball in the net moments later but the goal was rightly ruled out for his foul on Michael Parkhurst.
Nordsjaelland then assumed total control as Mikkel Beckmann sent a looping header just over from Okore's long ball, Petr Cech only parried Nicolai Stokholm's swerving 25-yarder and Beckmann screwed Parkhurst's cross wide.
Chelsea withdrew Moses for Eden Hazard and Nordsjaelland sent on Mario Ticinovic for Nordstrand and it was the home side's change which almost paid off when Cech brilliantly saved John's curler onto the post and Cole was on hand to block Beckmann's rebound.
The visitors hit back and Runje was lucky to see only yellow for a shocking tackle on Ramires, Oscar lashing the ball home after the whistle had gone.
Mata wasted the free-kick but when Chelsea got another 11 minutes from time, Luiz swerving a vicious shot in off the inside of the near post.
It was game over three minutes later when the champions' best move of the match ended with a lucky break off a defender, with Mata stroking home.
And there was still time for Ramires to put gloss on the scoreline by tapping home Oscar's cross.