The 20-year-old has featured in the Three Lions' warm-up games ahead of this summer's tournament in Brazil, despite having only played a handful of games for the Toffees, but Everton coach Stubbs is adamant that Stones would be ready if Roy Hodgson needs him.
He told talkSPORT: "We've seen it before, players who can only play in their position and sometimes you get a few injuries and then you're struggling.
"There's quite a few players in this squad who are flexible, which I think is vital when you're taking a team to a championship.
"You do need that flexibility and John Stones would provide that. Ability wise it's not in question, it's just whether he's ready for that big step up."
Stones has made 21 appearances for Everton since joining from Barnsley last summer.