Stokes was declared fit after nets on Friday to take his place if needed in an unchanged team as England bid for an unassailable 2-0 lead in the Royal London Series.
Also expected to keep his place at the Ageas Bowl is seamer Liam Plunkett.
At 32 it has taken Plunkett 12 years and 52 one-day international caps to begin to feel he is established in the first-choice XI.
After 18 appearances in England's last 22 ODIs, Plunkett appears a sure bet to keep his place against South Africa and for next month's Champions Trophy.
"I've played long enough now where I've been in and out of the side, but I feel in the last year I've been one of the better bowlers and one of the most consistent," he said.
"It is nice to hear people consider you an established member of the team, and I feel I do deserve to be in the side."
Plunkett has endured years rather than mere spells out of the England reckoning, and at one point his professional career was in danger of ebbing away too.
"It is fickle," he added. "Sometimes it changes for the better of the team - they might bring in an extra spinner or a left-armer.
"There are people wanting to come in ... so you need to keep improving. If you stay still, people are going to come in and do well.
"Little niggles, if you get rested for a game, someone comes in and performs and you could lose your place."
England's challenge is to win their next seven matches - a sequence they managed last summer - in order to both beat South Africa and then win their first global ODI tournament.
Plunkett believes they can do it.
"Absolutely," he said. "We are backing ourselves.
"It would be absolutely amazing to win a big tournament, and in England I think we've got a massive chance with this squad."
South Africa are determined to put the pressure back on England after their opening defeat.
Faf du Plessis admits the tourists, number one in the world rankings, were well short of their best and made "silly mistakes" at Headingley.
"We were disappointed with the way we played ... that's probably the most mistakes we have made in a game for a while," he said.
"But England put you under a lot of pressure, and you have got to make sure you are on top of your game and transfer the pressure back on to them."
