Southampton's hopes of securing a top-four finish in the Premier League were dented on Sunday when they suffered a 1-0 home defeat to Swansea City, with a fine long-range strike from Jonjo Shelvey in the 83rd minute proving decisive.
Koeman has previously revealed an interest in Feyenoord's Tonny Vilhena and Filip Djuricic, who is on loan at Mainz from Benfica.
And the Southampton manager is confident of bolstering his ranks as he seeks to guide the club towards UEFA Champions League qualification.
"We will have at least one player, I think we need that," said Koeman.
Pressed on whether Vilhena or Djuricic would be arriving at St Mary's Stadium, Koeman gave little away.
"I don't answer questions about names. But you will know it tomorrow," he said.
