And the Villa old boy, who helped bring both players through at Middlesbrough, believes the England squad is big enough for the both of them.
"I think Stewart will have watched closely what's happened to Adam Johnson over the last couple of months," said Southgate. "He should definitely see it as a challenge. Stewart went to a World Cup, he was involved with the England team under Steve McClaren for quite a while, but he's still very young.
"He's still got a lot of exciting years ahead of him and, as I always say with players of that talent, it's down to them how far they want to go really."
Southgate believes Downing and Johnson have different qualities - but insists the claret and blue star lacks the confidence of his younger colleague.
"Obviously, I had both of them at Middlesbrough," he told the Sunday Mercury. "They're completely different types of player and completely different characters.
"Adam probably has a bit more self belief than Stewart and Stewart now has to see what's happened with Adam, and make sure it galvanises him.
"There's no question Stewart has the quality to play for England. There were elements of his game that were better than Adam's, but Adam had certain skills and attributes better than Stewart's.
"The best season Stewart had with us he scored 10 goals, he created innumerable goals for other players and he really was an asset to us as a team.
"I do think the injury last year had a big effect because if you miss pre-season you never really recover from that."