When quizzed on the reasons behind Pelle's omission from the matchday squad, Koeman revealed that he had not been impressed by the striker in training.
"I wasn't happy about the intensity of Graziano in the training sessions this week," Koeman told reporters.
"I like to be straight with everybody: other players were more deserving of being part of the 18 today. He knows, and I will see what happens next week."
Southampton's victory takes them up to seventh in the Premier League table, just three points adrift of fifth-placed Manchester United, albeit having played a game more.