Alonso started only eighth after two mistakes in qualifying compromised his session and gained only one position to finish seventh in the race behind team-mate Lance Stroll.
This marked the second time in 2023 that he hasn't been on the podium for Aston Martin.
"We didn't have the pace, so that's the biggest problem," Alonso noted.
"It was not that we were unlucky or on differing strategies or anything like that, I think we were slow compared to the Mercedes – slow on the soft, slow on the hard.
"And we just concentrated on the Alpines, AlphaTauris, and kept up the pace with the Ferraris.
"At the end, I think we outscored the Ferraris this weekend because they only scored with Carlos [Sainz]. And we lost points with the Mercedes, but they have done a better job this weekend. So let's try the next one."
Asked if the team's form was track specific or related to upgrades brought by its rivals, Alonso admitted that both played a role.
"I think the track was a little bit not the best for us," he said.
"From FP1, we were on a completely different set-up from what we predicted, so we had to work a lot during the weekend to be happy with the car, which was kind of new for us.
"We were always happy from FP1 [this year], while here we had to work a little bit more.
"And then the upgrades from other people, I think maybe they didn't show their full potential in Monaco and Miami – street circuits. So here, we saw today that maybe they are a little bit stronger.
"But I'm not worried. There are going to be better weekends and worse weekends, and we will have our opportunity in Canada."
Asked by Motorsport.com if Barcelona was a reality check after such a strong run of races, he said: "No, I don't think so. I think in two weeks' time, we will see a completely different picture and hopefully, we will fight with the Red Bulls soon."
He added: "I need a good qualifying. I regret my mistake from yesterday, I thought all night if I could rewind and go again in qualifying, things would be different.
"That I cannot do yet, change time! So I can only think now of Canada and I will use that motivation for a good weekend there."
