Alonso has yet to qualify on the front row this season, and despite being quick in practice he was only able to take fifth on the grid for his home race.
"I think today was not a surprise for anyone in the team," Alonso said.
"We were three or fourth tenths off pole position in the last two or three grands prix, and here some people brought new parts so we are half a second off.
"We need to recover this half-second."
But Alonso has plenty of faith in the Ferrari in race trim.
"The car on one-lap pace is not as quick as the others," he said.
"But on the long-run pace it's as quick as the others, or even better.
"That is our strong point and we need to take every opportunity that comes tomorrow.
"Tomorrow we need to be optimistic and try to be on the podium, or even something better.
"We have the car to do a good race."
When reminded that the Spanish GP is traditionally won by a front-row starter, Alonso replied: "It's time to change this record..."
He suspects that Mercedes duo Nico Rosberg and Lewis Hamilton - who filled the front row - will fall back in the race, making second-row starters Sebastian Vettel and Kimi Raikkonen the main opposition.
"Every race is a unique race. But if you look at races in this championship, we see that Mercedes is struggling a lot in the races," said Alonso.
"Kimi and Sebastian may be more of a target for the race itself than the Mercedes guys, and also in the championship Sebastian is first and Kimi is second.
"It would be nice to finish the race in front of them. And I think if you finish in front of them, you will be very close to victory here."
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