Arsenal claimed a hard-fought 1-0 win over Crystal Palace at Selhurst Park despite playing the final half-hour with 10 men.
Nketiah should have done better with his second scoring opportunity, as Declan Rice played him through on goal and he lifted his chipped effort over the bar.
Martin Odegaard surprisingly stepped up to take the spot-kick instead of last season's penalty-taker Bukayo Saka, and sent Johnstone the wrong way. It was the first time Odegaard had taken and converted a penalty in a league match since May 2019, when he was at Vitesse Arnhem in the Eredivisie.
Arsenal’s resolve was tested as Takehiro Tomiyasu was dismissed for two yellow cards in the space of seven minutes.
The first was shown by referee David Coote after the defender took too long over a throw-in, and the second was given in the 67th minute for a foul on Jordan Ayew. Tomiyasu became the fifth player to be sent off in nine Premier League matches this weekend.
The visitors had a scare when Eberechi Eze showed wonderful skill to glide past two Arsenal players before going down in the box under a challenge from Thomas Partey, but he was not awarded a penalty.
Arsenal had to endure seven minutes of stoppage time but held on for the victory, their first on a Monday night in five attempts, to make it two wins out of two.
Roy Hodgson was left to reflect on his first home defeat since returning to Palace in April, after a five-match unbeaten run at Selhurst Park.