Michael O'Neill says he is mystified as to why Serge Gnabry did not make more of an impact in English football after watching the Bayern Munich forward bag a hat-trick as Germany beat Northern Ireland 6-1 in Frankfurt.

Gnabry moved on to 13 goals from as many international appearances, looking a world apart from the player who struggled to make an impact at Arsenal early in his career and virtually disappeared on loan at West Brom before being sold to Werder Bremen for a reported five million euros in 2016.

His first goal was the pick of the bunch as he took a touch before swivelling in the box and unleashing a shot high into Bailey Peacock-Farrell's goal, levelling the scores at 1-1 after Michael Smith gave Northern Ireland a shock early lead in this Euro 2020 qualifier.

But Gnabry's movement, pace and strength caused Northern Ireland's injury-hit defence problems until he was withdrawn nine minutes from time, leading O'Neill to describe him as "unplayable" on the night.

"Gnabry's goal-scoring record is phenomenal," said O'Neill, whose side now face a play-off against Bosnia and Herzegovina next March. "It's a mystery to me how English clubs didn't get more out of him.

"I'd be surprised if this Germany team doesn't make a big impact (at Euro 2020)."

This was Gnabry's second international hat-trick, and came less than two months after his four-goal display during Bayern's 7-2 demolition of Tottenham in the Champions League.

There were precious few signs of that sort of form during an Arsenal career that yielded only one goal from 18 appearances, but Germany coach Joachim Low said he had long been aware of the 24-year-old's talent.

"I've always had the highest possible opinion of Gnabry," Low said. "Right from the start it was obvious to me he would be a fantastically important player.

"He was always one to pass the ball to, he doesn't play up front all the time, he lets himself fall back a bit and you can find him anywhere on the pitch and he's able to play in many different positions.

"These are prime qualities in any players and also his finishing is technically fantastic."

Low put Gnabry's difficulties in England down to injuries, but said the forward's Arsenal team-mates Per Mertesacker and Mesut Ozil had been quick to recommend him.

"When he was at Arsenal he was plagued by injury and all these injuries set him back," Low said. "But even at the time, around the 2014 World Cup he had always been in my thoughts.

"He had some very good games with Arsenal and Mertesacker and Ozil would say to me, 'Look at Gnabry, he's a fantastic player, he keeps defences very busy', so we had him in our sights for a long time."