Pep Guardiola believes that the referee had no choice but to send off Wojciech Szczesny in Bayern Munich's 2-0 win over Arsenal on Wednesday.
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.
David Alaba missed the resulting spot kick, but the outstanding Toni Kroos and substitute Thomas Muller scored in the second half to give die Roten a 2-0 triumph in the Champions League last-16 first leg.
"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 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 attempt a repeat of last season's 2-0 victory in Munich.