England are further affected by the loss of all players from Leicester, Wasps and Bath who are in action in European finals a weekend before the first Test in Bloemfontein on May 26.
Jason Robinson will captain a new-look England squad, which includes seven uncapped players and the return to international rugby of Bristol hooker Mark Regan and Saracens prop Kevin Yates.
But England coach Ashton refused to dwell on frustrations which are beyond his control with the tour presenting opportunities for the likes of uncapped Bristol prop Nick Wood and Harlequins full-back Mike Brown.
Ashton said: "We have two Test matches to play and this is a golden opportunity for players to show they can handle Test match rugby in the heat of Bloemfontein and Pretoria.
"We will look to put in a challenging performance and we have one or two characters in the side who revel in that situation.
"It is going to be a massive challenge and incredibly tough but on a one-off occasion it has been known for sides to win when they are very much underdogs.'
Ashton was still able to draw on five World Cup winners in the form of Robinson, Regan, Jonny Wilkinson, Andy Gomarsall and Ben Cohen.
In the absence of Catt and Phil Vickery, who will have been on European Cup final duty for Wasps against Leicester, Ashton opted for Robinson as captain ahead of Wilkinson or former skipper Pat Sanderson.
He explained: "I went for Jason because he has had more experience on the field of captaining an international side.
"Jonny has never set foot on the field as captain of England and Pat Sanderson has only done it twice.
"With the squad we are taking out there we needed someone who has done it before. He is a player who is massively respected by all the other players in the team."