England beat the Proteas by 19 runs at Cardiff on Sunday to seal a 2-1 series win, with Morgan's fellow Middlesex left-hander Dawid Malan, who got his chance because of the skipper's absence, making 78 on his Twenty20 debut.
But following the toss, with Buttler leading the side instead of Morgan, former England captain Michael Vaughan tweeted: “1-1 ... Series decider ... and the England Skipper is resting !!1 #WTF ..... Worlds gone mad.”
1-1 ... Series decider ... and the England Skipper is resting !!!!!!!!!!!!! #WTF ..... Worlds gone mad— Michael Vaughan (@MichaelVaughan) June 25, 2017
And former England batsman James Taylor, commentating on BBC Radio, said of Morgan: "He's the leader and he should be playing."
Morgan is now a limited overs specialist at international level and so is unlikely to be involved in the upcoming four-Test series against South Africa.
But Buttler defended Morgan's move by saying: "It shows how much cricket gets played.
"It's very important to keep guys mentally fresh," added the wicket-keeper, who had previously led England in Dubai and Bangladesh -- a tour that Morgan missed because of security fears -- but never before on home soil.
Also Read: Eoin Morgan Defends Dropping Himself for South Africa Finale
Buttler rejected suggestions that spectators in Cardiff had been short-changed by Morgan's absence.
"If you turned up here, saw a brilliant game of cricket and England win the series, then as an England fan I think you can go home very happy," he said.
- 'Development' -
Morgan previously justified his move by saying: "We recognise the series as a big opportunity to have a look at a younger group of players.
"If it was a case where I could go on and captain, I would. But this is an important part of our development for this series."
Malan was named man-of-the-match after posting the highest score by an England cricketer on their Twenty20 debut.
The 29-year-old was born in London but brought up in South Africa, where he made his first-class debut for Boland.
"Dawid showed by that performance that there are some good players really knocking on the door," said Buttler.
"It's not an easy thing to do, but he showed great composure on a wicket which he made look a lot easier than it was.
Also Read: 3rd T20I: England Beat South Africa by 19 Runs, Take Series 2-1
"He looked as if he was playing his 50th game, as opposed to his first."
Meanwhile Surrey quick Tom Curran, in just his second match at this level, took two for 22 following three wickets on debut at Taunton on Friday.
And Sunday's match also saw 20-year-old Hampshire leg-spinner Mason Crane dismiss one of the world's best batsmen in South Africa captain AB de Villiers for his first wicket in international cricket.
"I was impressed with Tom again," Buttler said. "He bowled well at Taunton and he knows his game very well, as does Liam Plunkett.
"Mason also seemed very composed. De Villiers obviously targeted that over to put pressure on a young guy in his second game.
"But Mason showed great heart and character to get his wicket."
First Published: June 26, 2017, 7:22 PM IST