Janmaat said the side shot themselves in the foot.
The Magpies were up against it from the very first minute of their visit to the Champions when Vurnon Anita conceded a penalty by tripping Edin Dzeko.
It was the start of a very long evening with City racing into a 3-0 lead in the first half before a quick-fire brace from David Silva rounded off the scoring towards the start of the second half.
Janmaat told the Chronicle: "We say sorry, we have to apologise because it’s not good enough.
“To come here you know it will be a hard evening.
“If you give a stupid goal away in the first minute you know it will make it even more difficult.”
The frustrating things for Newcastle fans will be how abject the performance was having beaten City at the Etihad Stadium earlier on in the season in the League Cup, where they caused Manuel Pellegrini's side so many problems by pressing high up the field and staying compact and tight at the back.
As good as Manchester City were - who were boosted by title rivals Chelsea dropping points before kick-off - Newcastle helped them on their way, as the Dutch full-back admitted.
He added: “We shoot ourselves in the foot.
“It’s stupid. There are no positives."
It is a blow for Caretaker boss John Carver, who said the side had a game plan - but it went out of the window almost immediately, and the response was poor. He can't afford many more poor performances from his team if he wants the job full time from next season.