Many were surprised to see Hein in the starting XI for the third-round match of the EFL Cup at the Emirates. The 20-year-old made his senior debut as Arteta chose to play him over second-choice goalkeeper Matt Turner.
It didn't go quite as well as Hein would have hoped as the Estonian fouled Danny Welbeck in the box, giving away a penalty. The former Arsenal striker scored from the spot to cancel out Eddie Nketiah's opener.
Kaoru Mitoma and Tariq Lamptey each scored a goal in the second half to eliminate the Gunners from the EFL Cup.
After the match, Arteta backed Hein and admitted that mistakes are a part of the game but you learn only when you play. He said (via Football.London):
"I think he deserves the chance. He’s the first goalie of the national team and at his age, that’s not usual. He totally deserved his chance. If I have to play him tomorrow again I would.
"You get experience when you play football matches and if not you never get the experience, but Karl already has that. Errors are part of football."
Hein has played 40 games for Arsenal's youth sides and has also earned 15 caps for Estonia.
Mikel Arteta reveals Karl Hein has gotten support from Arsenal teammates
Arteta was also asked if he spoke to the youngster after the game or is letting him deal with the mistake himself. The Spaniard said:
"No, everybody has been supporting him, everybody was supporting him straight after. We all make mistakes, me the first one, the players all the time and it’s part of the game. He needs to carry on.
"When you make an error it’s about how you react, not about the error. This is going to happen in his career and it happened today, unfortunately."
The defeat to Brighton was only Arsenal's third defeat of the season after 20 games in all competitions. They earlier lost to Manchester United in the Premier League and PSV Eindhoven in the Europa League.
The Gunners sit at the top of the Premier League table with a two-point lead over second-placed Manchester City. Arsenal will face Wolverhampton Wanderers away on Saturday, November 12, in their final game before the 2022 FIFA World Cup.