Jim Mallinder says London Irish’s win at Saracens earlier in the season acts as a warning for Saints.
Mallinder’s men meet Irish at Franklin’s Gardens on Sunday, hoping to enhance their chances of a top-two finish in the Aviva Premiership.
Irish have little to play for, having enjoyed a mixed season in which they have refused to flirt with relegation.
But Brian Smith’s men have shown what they are capable of, winning 22-13 at Saracens back in February.
And Saints boss Mallinder said: “They’re a real dangerous team. You don’t quite know what you’re going to expect from them.
“They’ve probably had a disappointing season in terms of where they are and they probably haven’t got much to play for apart from pride and looking forward to next season, but they’re very, very dangerous.
“They’ve put in some great performances. They played very well against Newcastle last weekend and they’ve been away and beaten Sarries. Any team that can do that can turn anyone over on their day.
“We’ve got to be organised, motivated and if we do that hopefully we can get the win we need.”
And Mallinder is urging his team not to even give Irish a small scent of victory.
He added: “They can play rugby and they do like to move the ball. They’ll come here and move the ball, whatever the conditions. They’ll look to play with some real width, get the wingers and full-back involved.
“I know they’ve got a few injury doubts and they’re talking about next season but that won’t stop them being positive. It’s the way Brian Smith and his teams always play.
“Give them a sniff and their heads are going to go up, which would make it a very hard afternoon.”