After Gnabry made it 3-1 just a minute into the second half as Germany ran riot with Goretzka adding a second before Julian Brandt's strike in stoppage time.

"It was a tough night," said O'Neill, who suffered his worst defeat since a 6-0 friendly loss to Holland in his second game in charge back in 2012.

"When you come into a game like this, when you're missing players, at the back of your mind you fear this may happen and unfortunately it did.

"We couldn't have got off to a better start, it was a great goal and we had some good pockets of play in the first half.

"The killer really is we're a minute from going in at half-time at 1-1 and by the 47th minute we're 3-1 down, and that's down to the quality of the opposition and a little bit of poor defending.

"Once we're 3-1 down it's difficult to get energy into the team."

The gulf in class was apparent in every department, but most obviously in Gnabry who moved on to 13 goals in as many international games.

"Gnabry was unplayable tonight," O'Neill said. "Fantastic movement, fantastic finishing. For us, a double-header against the Netherlands and Germany without a full-strength squad, we don't have the depth to be able to deal with it."

The win confirmed Germany as Group C winners, and the manner of it suggested the young side Joachim Low is moulding can look forward to next summer's finals with some optimism.

"What I liked a lot was that after being 1-0 down with the very first shot at goal we were immediately playing up front and playing offensive football, and at no point did anybody really feel we were going to lose this," Low said.

"I think we played for the full 90 minutes with full concentration and I think the team rewarded themselves for all the effort they put in.

"For the full 90 minutes we kept trying to score goals and we did score goals so that was really fun watching them play tonight."

Northern Ireland knew before kick-off they were heading to the play-offs, and knew by full-time their opponents will be Bosnia and Herzegovina away from home thanks to Wales' victory over Hungary.

O'Neill's men lost home and away to Bosnia in the Nations League though were unfortunate to do so, picked off on the counter-attack in Belfast and then hitting the post three times in a 2-0 loss in Sarajevo.

Much has changed for Northern Ireland since and O'Neill, who plans to remain in charge until March alongside his new role as Stoke boss, believes they can approach the trip with a degree of confidence as long as they have their key players available.

"The fact we won't play Germany is the first thing," he said. "We played Bosnia in the Nations League and we should have beaten them at home and we were unfortunate to lose in Sarajevo so the experience of being there will help us.

"We played tonight and probably had three of our first-choice back four missing, with Jamal and Jonny and Stuart Dallas who played in the game against the Netherlands, and ultimately against this level of opposition, unfortunately we weren't able to deal with it."