Alonso ran wide through the gravel on his out-lap at the start of the session, damaging the floor sufficiently to make the car tricky to drive for the remainder of qualifying.
Thereafter, he noted that it handled "weirdly" but the Spaniard remains convinced that he was still on target for the front row before he made a mistake and ran wide on his final lap.
"P2, probably even with the floor as it was," the Aston Martin driver said. "Because in the Q3 lap I was coming for 12.7s until Turn 10. Then again I ran wide into the damp part in Turn 10 on the outside, so when I saw that 12.7 is P2 and P3 I was surprised.
"That's why I'm optimistic for tomorrow, because the car seems to have a lot of pace."
His mistake meant Alonso had to settle for ninth on the grid for his home race.
"Q1 probably did compromise everything today, I did a mistake," said Alonso. "I went on the damp part of the circuit, I guess, on the last corner, because I lost the car.
"And then it was very costly because that gravel completely destroys the floor. And yeah, it hurts even more because it was the lap.
"I was not even pushing, it wasn't a push lap, it was the out-lap. So I'm disappointed with my performance today, hopefully I can do better on Sunday."
Asked by Motorsport.com if he could feel a loss of downforce, he said: "The car was moving weirdly in few corners, but you never know if it's just the wind or just the track conditions.
"It was a strange qualifying to see [Charles] Leclerc out of Q1, and [Sergio] Perez, [George] Russell out of Q2. We were struggling as well. It was tricky for everyone, but in my case it was just my mistake in the lap of Q1, so that compromises everything.
"But the race is tomorrow, the car still feels competitive even with some damage on the floor. So if we put everything together for tomorrow I'm still optimistic that we can score many points."
He added: "I would say top five, top six has to be possible from P9. I realised that Checo is starting at the back, so it's like a starting P10, not P9 because he will pass by very fast, and then from that point, we need to recover four or five places. It's going to be tricky but we will do our best."
