The ex-Barcelona boss thinks the referee made the right call in Arsenal's 2-0 defeat to Bayern Munich and was delighted to win his first game at the Emirates in his third attempt
The Gunners started strongly but a missed Mesut Ozil penalty and the sending off of their goalkeeper took the wind firmly out of their sails.
And Guardiola thinks match referee Nicola Rizzoli had no choice but to send the Poland goalkeeper off.
"Arsenal were much, much better than us in the first 10-15 minutes but, after the mistake from Ozil, from that moment we played better," Guardiola told Sky Sports.
"After the red card it became our game. If it's the last man then the referee has to give a penalty and a red card."
The former Barcelona boss lamented the Gunners' defensive approach after they went a man down but was pleased with the way his team wore the hosts down - and cherished his first win at the Emirates Stadium at his third attempt as a coach.
"It's difficult to play when you see nine players in their box," Guardiola added. "It was important that we controlled the situation. We played with patience, patience, patience and in the end we got our goals.
"The last two times I came here with Barcelona we played amazing but we didn't win. So it's great to win here.
"[We must avoid what happened] last year when Bayern had a bad result in the second leg. We're going to play the second leg to win the game."
The Allianz Arena will host the second leg on March 11, when Arsenal will attempt a repeat of last season's 2-0 victory in Munich.