England slumped to a 68-run defeat to India on Thursday, as their title defence came to an end.
Buttler's team were skittled out for just 103 in Guyana, as India gained revenge for their 10-wicket loss at the same stage two years ago.
It has been a tournament of ups and downs for England, who scraped through the group stage and had to recover from a loss to South Africa in the Super 8s to reach the last four, but India's quality proved too much to handle.
"India outplayed us, certainly. We let them maybe get 20 or 25 too many on a challenging surface that they played well on. They fully deserved to win," Buttler said.
"With everything that's happened in the tournament, I'm really proud of the group's efforts.
"We can only play who's in front of us. We've had a lot of adversity but we have stuck together really well, we played good cricket in patches but unfortunately came up short when we needed it most."
England won the toss after a rain delay and put India in to bat, though Buttler does not feel that was an error.
"With the rain around, we didn't think the conditions would change and they didn't, they just outplayed us and got a very good score," Buttler said.
" I don't think the toss was the difference between us.
"They've got some fantastic spinners. Our two bowled well but in hindsight, yeah, probably should've bowled Moeen [Ali] in our innings with the way spin was playing.
"But with a good score and their brilliant bowling attack, it was always going to be a tough chase."
