There could hardly have been a worse start for Newcastle as they fell behind in only the fourth minute, after some fine individual skill by Shane Long – and some hapless defending.
Long, returning after a knee injury, waltzed effortlessly past half-hearted tackles by Vurnon Anita and Steven Taylor, before clipping the ball past keeper Karl Darlow.
The visitors might have conceded a second when Taylor’s back pass lacked conviction. But Darlow to just managed to beat Long to the loose ball.
And it got even worse for Benitez’s side in the 38th minute as they fell further behind.
Daryl Janmaat failed to cut out Sadio Mane’s crossfield pass, allowing Dusan Tadic to race away down the left.
The Serb had time to hit a controlled cross to Long, who neatly set up Graziano Pelle to fire past the exposed Darlow.
Janmaat, who slipped awkwardly as he missed Mane’s pass, was left nursing a groin injury and was immediately replaced by Siem De Jong.
Taylor made way for Jamaal Lascelles for the second period but Saints were three up in the 55th minute, as Jose Fonte’s shot from Mane’s cross was blocked, only for Victor Wanyama to hammer in the rebound.