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.
The error meant Alonso had to settle for ninth on the grid for his home race, as Max Verstappen secured pole for Red Bull.
"P2, probably even with the floor as it was," the Aston Martin driver said when asked what he believed would have been possible.
"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."
Alonso conceded that his Q1 off "probably did compromise everything today".
Explaining the incident, the 2006 and 2013 Spanish GP winner said: "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."
Source: Autosport