Wayne Routledge says Arsenal's 8-2 humiliation at Manchester United sends a clear warning to Swansea City of the Premier League's merciless nature.
Swansea travel to Arsenal on Saturday seeking their first win and a first league goal of the season.
And Routledge, who has previous top-flight experience, warns that even the top teams can suffer on a bad day.
"The Premier League is probably the most unforgiving league in the world," said 26-year-old Routledge.
"In the Championship most teams beat each other but if you're not on your game in the Premier League you can get taken apart.
"You always have to be on your game and play to the best of your ability. We need to play our game and we'll play our game on Saturday."
Swansea have lost one and drawn two of their opening three games in the Premier League.
Routledge, who joined the Swans on a three-year deal from Newcastle in August, has featured in all three games.
And although the Welsh side are yet to score, former Tottenham player Routledge says Swansea can be pleased with their start to the campaign.
"The feeling is we've had a decent start," he said. "It's very early in the season and the league hasn't panned out yet.
"But I think we've showed in the first three games we're capable of matching teams in the Premier League.
"We've got two points from a possible nine but in the whole scheme of things we've had two clean sheets.
"For a promoted team to keep a clean sheet is hard enough so to keep back-to-back clean sheets is a big boost for the team."
Manager Arsene Wenger has responded by signing Mikel Arteta from Everton for £10m and Chelsea's Yossi Benayoun joining on a season-long loan.
"If you lose quality you've got to replace it with quality and I feel they've done that," added Routledge.
"It will be a very, very good entertaining game of football on Saturday."