Another injury-disrupted campaign means Wilkinson has not played Test rugby since England's crushing RBS 6 Nations defeat against Ireland at Croke Park in February.
But Ashton will require every ounce of the Newcastle fly-half's experience as a severely-depleted England squad prepares for demanding back-to-back appointments with South Africa in Bloemfontein and Pretoria.
Ashton is due to reveal his first Test line-up on Tuesday, when a handful of uncapped players could feature either on starting duty or among the replacements.
Gloucester prop Nick Wood, Saracens hooker Matt Cairns, Bristol lock Roy Winters and Sale Sharks second-row forward Dean Schofield will be among those eagerly awaiting Ashton's announcement.
But World Cup winner Wilkinson's presence will be critical at Vodacom Park in Bloemfontein next Saturday when England hope to disprove the popular theory their two-week trip is little more than a damage limitation exercise.
Wilkinson could forge an all-Falcons midfield trio alongside Toby Flood and Mathew Tait, and Ashton said: "Jonny brings a massive amount of experience, very high skill levels and great enthusiasm for the game.
"He is a player that relishes a challenge, whether it is a personal one or a team one, and he won't have faced many bigger challenges than this in his rugby playing career."
As for the players at his disposal, Ashton added: "I have not heard anybody say it is a fourth XV.
"It was my decision to leave out the (Leicester, Wasps and Bath) players involved in the European finals, so this group is the next available group of players.
"There are a lot of very proud people in the squad, and they are going out to pull on an England shirt when maybe they thought the possibility had passed them by.
"Prior to the tour squad announcement, one or two of them were on the phone volunteering their services. Players like that are not going to lie back in the face of any challenge."
England's 30-man squad continued their build-up in the Johannesburg suburbs, where the early-morning temperature barely rose above freezing due to a weather front expected to remain in the Gauteng region for another five days.