William Saliba was unable to participate in Arsenal’s penalty shootout against Manchester United in their FA Cup third-round clash due to a rarely-discussed regulation.

Arsenal bowed out of the competition after a thrilling shootout defeat to the ten-man Red Devils at the Emirates, following a 1-1 stalemate during regular and extra time.

Manchester United emerged 5- winners in their FA Cup penalty shootout against ArsenalCredit: AP

Gunners defender William Saliba was unable to take a penalty due to a little-known ruleCredit: GETTY

Mikel Arteta’s men dominated the match but were unable to find a winner – despite having an extra man due to Diogo Dalot’s second-half red card.

And Dalot’s dismissal resulted in Saliba not being able to take a spot kick.

FA Cup rules prohibits a team from having more available penalty takers than their opponents in a shootout.

That meant one of the Gunners had to choose one player to step aside ahead of the shootout, with France international Saliba the sidelined star.

United ran out 5- winners in the shootout, with striker Joshua Zirkzee scoring the decisive penalty.

Kai Havertz missed the Gunners’ second penalty after Martin Odegaard made amends for his own spotkick miss in normal time.

Havertz’s failure to convert from 12 yards added to a catalogue of earlier misses.

And sick trolls ended up targeting the Germany international’s wife, Sophia, with vile messages after the final whistle.

Victory in London has earned United a fourth-round clash with Leicester City, who they’ll welcome to Old Trafford on February 8.

Red Devils boss Ruben Amorim said: “We deserved to win, Arsenal didn’t deserve to lose but we deserved to win.

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“The way we played, the way we fought was really tough since the beginning.

“We didn’t have the ball too much but we had control without the ball.

“I want to also say thank you to all the fans. It was unbelievable.

“I did not understand that all that area was our supporters, so it was a good day for us now we have to rest the players, I’m already thinking about the next game they’re really tired.

“Sometimes we have the feeling that everything in the end is going to be okay.

“I felt that since the beginning of the second half, even after the penalty. I think the Arsenal team felt that maybe today is not their day.

“I’m really pleased for the way they controlled the game without the ball. We defended very well, set pieces we were really strong.

“It was a good day but now I’m thinking about the next game and worried about the next game.”

Kai Havertz missed the crucial penalty for Arsenal