Manager Arsene Wenger says Arsenal "have to be patient" with new signing Danny Welbeck after his miss proved costly in the 2-0 Champions League defeat at Borussia Dortmund.
The 23-year-old England forward, a £16m arrival from Manchester United this month, shot wide when clean through with the Group D game goalless.
Dortmund scored either side of half-time to secure victory.
"Danny will improve - we don't have to make a problem of that," said Wenger.
|Former Arsenal striker John Hartson on BBC Radio 5 live:|
|"Arsenal papered over the cracks last season, winning the FA Cup and just sneaking into the top four. Their performances against the big teams in the last few years have been unacceptable. They could have conceded five or six against Dortmund. Without overreacting, it was a really woeful defensive performance from Arsenal."|
"He had a very lively first half. He had two or three good chances but couldn't finish them. There was one obvious one. We have to be patient with him."
Welbeck scored both goals as England beat Switzerland 2-0 in a Euro 2016 qualifier on 8 September.
But he has not found the net at club level since scoring for United in a 3-0 Premier League win at West Brom on 8 March.
After Welbeck's miss at Signal Iduna Park, Ciro Immobile gave the home side the lead with a low shot, before Pierre-Emerick Aubameyang finished another fine breakaway.
Aubameyang also hit the crossbar in a game that saw Dortmund have 22 shots to Arsenal's four.
Defeat in their opening game leaves the Gunners - competing in the Champions League for the 17th successive season - bottom of the group after Galatasaray and Anderlecht drew 1-1.
"It's very difficult to go into any individual assessment of any player because as a team we were not at the requested level," said Wenger.
The Frenchman said midfielder Jack Wilshere "turned an ankle" late on, although the 22-year-old finished the game.
"It's difficult to say how bad it is," Wenger added. "But because of his history I'm a bit cautious."
Arsenal face Galatasaray at Emirates Stadium in their next Champions League game on 1 October.