The Spaniard's Las Vegas weekend was compromised by the water valve incident in practice, and he was only able to secure sixth place on a day when team-mate Charles Leclerc fought for victory.
In Abu Dhabi, he had a big crash in FP2, and after a poor Q1 saw him start only 16th and gamble on two stints with the hard tyres. Waiting for a safety car failed to pay off, and he tumbled out of the points when he headed to the pits on the penultimate lap.
Sainz finished seventh in the world championship, only six points shy of fourth place, which he occupied before Abu Dhabi.
"I think it's been a decent year for me," he said when asked by Motorsport.com about his season. "I think we've had a more consistent year, we've shone a bit more.
"I'm not happy with these last two races, obviously. It puts a dark patch into my season where I think we've had a much stronger season than what these last two races show.
"And the way everything has gone wrong for me in these last two races has been quite dramatic in a way, but it is what it is. Sometimes a year goes like this.
"Normally the end of the season is my biggest strength and this season, for some reason, it hasn't been the case. So it's time to sit down, analyse and see what I could have done better for these last few races and come back strong."
Sainz admitted that seeing his team lose second place to Mercedes by just three points was frustrating.
"Honestly very disappointed, and obviously not happy, given how close it was in the end with the constructors' championship," he said.
"But we will have to sit down, analyse, see what we could have done better today and what was going on because clearly the pace this weekend and the overall feeling with the car and everything wasn't good."
Conceding that his Abu Dhabi race fell apart because an opening stint on the hard tyre didn't pay off for him, a lack of performance led Ferrari to switch strategy and gamble on a safety car to give Sainz a helping hand.
"We started on the hard, expecting the hard to help us do a one-stop," he said. "Again like we've seen many times this year, whenever we start on harder compounds with our car, we struggle a lot, and it was again today.
"We had nothing to lose starting 16th and we gave it a go, but in the end again it didn't work for us. The harder compounds at the beginning of the race with dirty air and the sliding just doesn't work for us.
"And once we saw that we had very little chance of scoring points, we left it out for a safety car, and it didn't work out also. We had to retire in the end with a PU issue so it's not like it would have changed much."
