The Spaniard, who won his home grand prix at Barcelona just a fortnight ago, qualified sixth, before slipping back to seventh during the race.
While Alonso did spend a portion of the race with reduced aero thanks to having a piece of Sergio Perez's McLaren stuck to his Ferrari's floor, he said the main problem was a general lack of pace.
"The main problem was the pace," he explained.
"We didn't have the pace. Normally on Sunday we have the pace, but today we didn't.
"Apart from that, yes, we did have different problems. The team told me that we had a plastic bag stuck in the front wing for 10 laps, that took away performance from the front. And we had a piece of Sergio's front wing under the floor, and lost around 30 or 40 points of performance.
"But in the first 30 laps, before the red flag, we didn't have a [specific] problem. We were too slow."
Alonso also explained that he was thinking of the championship during the race, which led to him taking less risks while battling other drivers for position.
"When you fight for positions with people who have two or three points in the championship, there is a risk.
"I did the same in 2008 and 2009, when I had nothing to lose. I was overtaking in some strange positions, and it sometimes works. And if you have a broken front wing, you don't lose anything.
"[But] we're fighting for the championship, so we look in the mirror if someone tries to overtake us.
"Why did we find ourselves in that position? Because we didn't have the pace today and we didn't have the pace in qualifying.
"If you start at the front, you fight with the top guys. If you start in seventh or eighth, you fight with guys who have nothing to lose."
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