After slumping to a surprise 1-0 home loss against Swansea City last weekend, the south-coast club appeared set for another disappointing result as they were frustrated by a managerless but resolute QPR side.
But Senegal forward Mane powered home the decisive goal in the third minute of injury time at Loftus Road to boost Southampton's UEFA Champions League hopes.
"Always in a season you have some key moments and one of these was today, after the defeat, to show our way how we like to play football and have a good result," the Dutchman said.
"It's the fourth [league] win away in a row and that's fantastic."
Southampton are now third in the table and only four points behind defending champions Manchester City in second. However, Koeman insists his side's first goal is a place in the Europa League.
"It's not all about the first four places on the table, it's trying to reach a European position for next season," he added. "That will be the target.
"I look more back than up. Maybe in three weeks it will be different but, at the moment, we are watching the teams around us and behind us."
Koeman also confirmed a head injury suffered by Matt Targett, who came off on a stretcher midway through the first half, is not serious.
"He's [Targett] under control. That's very good news," the Dutchman said. "The message and the comments of the doctor was that he is under control in the hospital."
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