Nelson’s close-range finish from Alexandre Lacazette’s cross came 10 minutes into the second half. It was the 20-year-old striker’s second goal on his 29th appearance for Arsenal after being among four changes from the side that beat Manchester United last week.
Leeds out-shot Arsenal 15-3 and dominated possession in the first half and demonstrated why Marcelo Bielsa’s side is well placed to end a 16-year exile from the Premier League.
“The way they play they make it very difficult, they play man-to-man around the pitch,” Arsenal manager Mikel Arteta said. “If you are not ready for that game you get exposed.”
Arteta told players he has managed for only a couple of weeks that they weren’t reaching the standards expected of them.
“I cannot be happy and I have to let them know,” Arteta said. “We had to react and we changed some things.
“But the biggest change was how we approached the second half, the mentality, how aggressive we were, how we played in their half, won every duel, making runs forward. It was different.”
The draw for the fourth round was made before the game at the Emirates Stadium. Arsenal goes to Bournemouth next on the weekend of Jan. 25-26.
Arsenal is enduring a lackluster Premier League, sitting in 10th place after struggles that prompted the firing of Unai Emery as coach.
Leeds’ priority is clinching one of the two automatic promotion spots in the Championship, which the northern club leads with a nine-point cushion on third place.