Teenager midfielder Reed made his first Premier League start in the 3-0 win over Everton last weekend.
The 19-year-old had an impressive impact as the south-coast club ended their five-game losing streak in style at St Mary's Stadium.
Koeman felt that Reed deserved his chance and says his inclusion should send a message to Southampton's academy players that they will be given an opportunity if they prove they are capable.
The Dutchman said: "If I have confidence in a player, the player has possibilities to play. He starts in the team. That means a lot for the player. It means the coaching staff have confidence in the player.
"It was a good moment to bring Harry in, with his energy he gave to the team it was fantastic. It was a good position to play, we had some injuries and suspensions, it was the right moment for Harry.
"For young players it's good to have chances for that, the whole academy they see you get chances if you're doing well and working hard.
"We believe in our young players, it's part of the philosophy of the club. it was great to see."
Following the win over Roberto Martinez's side, Southampton are fifth in the table.
