The Saints look the surprise package of the campaign as Koeman's side sit third in the table behind Chelsea and Manchester City after winning five of their first eight games.
Their momentum shows no sign of slowing down after thumping Sunderland 8-0 last week, and the Dutchman admits that a top-four finish is not beyond reach.
"It looks like Chelsea and Man City are the strongest, but we'll see. Maybe we can [challenge for the top four] because if you believe everything is possible," he told Sky Sports News.
"We can be up there, but the difference is that top teams have 24, 25 players already prepared to play Premier League, and teams like Southampton don't have that.
"We have some young players who maybe will get the chance, they are talented players, but they don't have the experience. The biggest teams, they have so much more players on that level and that's the difference - but we'll see."
Southampton will look to maintain their stellar form when they take on Stoke City at St Mary's this afternoon.