Buttler is widely regarded as one of the best white-ball batters in the world and has racked up 161 ODI appearances but only 19 of those have been in the three and half years since England won the World Cup.
Existing mainly on a diet of T20s recently - where he opens the batting instead of the middle-order role he performs in ODIs - Buttler admitted even a player of his calibre has to take time to adjust.
He anchored England's 342 for seven with 94 not out off 82 balls but South Africa overhauled the total with five wickets and as many balls to spare to move into an unassailable 2-0 lead in the series.
"We're not quite playing as well as we can," he said. "Even myself, I've played a lot of 50-over cricket, I feel not quite used to the rhythm of the game. It's more with the bat.
"I've spent a lot of time recently playing T20 cricket at the top of the order and after 10 or 15 balls you're ready to push the accelerator. In 50-over cricket that's fine and you catch up and accelerate.
"Maybe it's batting at five now. I've spent a lot of my career batting six and coming up the order if we've started really well. That's for me to work out and remember and reconnect with things that work."
There are several mitigating factors for their current slump, not least bulging schedules that has seen ODI cricket at the bottom of England's priority list since they lifted their maiden world title.
England are frequently without some of their best players but Buttler was at pains to state he was not looking to make excuses and that improvement is a must ahead of their World Cup defence this year.
"We've got to move with the times, the schedules are how they are," he said. "We just need to play a little bit better.
"There's ingrained confidence in the guys over a long period of time in white-ball cricket. But definitely it's changing times as well, we've got to look forward."
Buttler, though, was upbeat despite Sunday's defeat in Bloemfontein, where Harry Brook helped lead England's recovery from 33 for two with 80 from 75 balls - his maiden ODI fifty after a duck on debut.
Moeen Ali registered his first ODI half-century since September 2017 while Olly Stone consistently threatened with the ball and was perhaps unfortunate to finish with figures of two for 48.
"There's plenty of positives to come out of this," Buttler said. "In the bigger picture of building towards a World Cup, Harry Brook confirmed what we already knew - he's incredibly special.
"I don't think he's played a 50-over game for three and a half years or something (before Friday). He was asking how he should play. I said, 'just play like it's a Test match!' That's pretty much how it goes for him.
"He's a great talent. He's got all the game to play in every scenario and all the formats."
Stone has been held back until after the powerplay in both matches in Bloemfontein, bowling at high pace and unsettling batters with bounce, prompting comparisons with Liam Plunkett.
Plunkett was key for England in the years leading up to and during their World Cup triumph in the middle overs and Buttler believes Stone has the tools to emulate the former Durham and Yorkshire quick.
"Liam Plunkett had tremendous success in that role," Buttler added. "He was a brilliant wicket-taker.
"Absolutely you're trying to find guys who can have something a little bit different in those middle overs to be able to find wickets, whether that's extra pace or trying to find ways to get the ball to go sideways.
"I thought Olly bowled brilliantly here and credit to him for that performance."
Temba Bavuma's position as South Africa white-ball captain has been under scrutiny after last year's elimination in the T20 World Cup group stages but he underpinned their successful pursuit of 343 with 109 off 102 balls.
"It was a reminder to myself that I deserve to be in this team," he said afterwards.