Newcastle won in London 2-0 in the first-leg on Tuesday night.
Arteta said afterwards: "That was the biggest difference (being clinical) in the game. In every other aspect, we were the better team.
"They scored two goals and credit to them as they managed to win the game like this.
"When we missed the chances and conceded the goal it is always tough, but the team reacted.
"We generated a lot of situations, but it is true, they defended the box very well.
"We can do better with our decisions.
"We had a one-v-one chance with the keeper and in the semi-final you have to be clinical."
On Alexander Isak's goal, he continued: "I have seen the goal and it is something they do - they create the chaos, and it gets to the wrong player and Isak puts it away.
"We have to be disappointed as it's another result that we wanted, but that's the reality. I have full belief. I have seen my team playing against them - I believe we can do it."