Fast bowler Steyn is out of the match after breaking his thumb on the first day of the tour game against Bangladesh 'A' which ended in a draw at New Road here Sunday.
Steyn took seven wickets in the Proteas' 10-wicket second Test win at Headingley which put them 1-0 up in the four-match series with two to play after the drawn opener at Lord's.
His loss is a blow to South Africa as they seek a first series win in England since their re-admission to international cricket.
But in fellow quick Nel, someone who often attracts as much publicity for his verbal spats with opposing batsmen as he does his bowling, Arthur is confident the Proteas have an ideal stand-in.
"Any time Andre plays for South Africa he's fired up. I can't wait to see him bowl hard. I think he's going to do really well," Arthur told the BBC.
"We've backed Andre a huge amount over the last couple of months and I've got a feeling he is going to come through this one fantastically.
"It just shows that when you plan there has to be a queue and some succession in place.
"We have the ability now to lose one guy and be able to bring back a guy who is a hardened Test-match campaigner. So we are fortunate to do that, but that's how we structured our team."
Nel, who has taken 119 wickets in his 34 Tests, had a comfortable warm-up for his return to centre stage with four wickets and the 31-year-old also weighed in with a maiden first-class fifty against Bangladesh 'A'.
South Africa, who didn't enforce the follow-on, saw AB de Villiers - whose 174 at Headingley helped provided the platform for victory - and squad batsman Robin Peterson both make hundreds against Bangladesh in a second innings total of 328 for four declared.
All-rounder Jacques Kallis, 55 not out overnight, did not resume his innings Sunday after sustaining an injury to the fourth finger of his right hand while attempting to take a catch.
However, he did field during Bangladesh's second innings and Arthur said Kallis was "100 percent fine" to face England.
Bangladesh, bowled out for 121 first time around, were 97 for two at the conclusion of this three-day match.