After netting just five times in 32 Premier League appearances in his first season at St. Mary's last term, the Republic of Ireland international has scored 13 goals in all competitions in a much-improved second campaign on the South coast.
That tally, at the age of 29, is Long's highest in a single season since he hit 25 goals for Championship Reading in 2010-11, and he has also struck three times for his country, including the crucial winner against world champions Germany in a vital Euro 2016 qualifier in October.
Long is the Saints' joint-top scorer in the league this season but boasts a better goals-to-games ratio than any of his teammates, and Koeman expressed his delight at the progress the former West Bromwich Albion star has made.
The Dutch boss said, as quoted by the Daily Mail: "I remember when Shane came, I think in all aspects he has learnt a lot. We did a lot with Shane in a lot of aspects of football and I think he is more complete as a player, as a striker.
"His pace, his timing, his movements are full of confidence. It is a big example that you can still learn when you are 26, 27, 28. He was always really excited to come to Southampton and I remember talking to Shane about training sessions, ours are a little bit different to the usual English striker.
"He is really complete as a striker, in the box he is clever, he has a really good header, he makes it so difficult for the defenders because he is always on the move. I am happy I played [as a defender] before Shane was a striker."
Long's goals, including three in his last four club matches, have helped the Saints launch a sustained challenge for Europa League qualification this season, one which the club can enhance with a strong points return from their final two fixtures.
With a trip to high-flying Tottenham Hotspur and a home clash with FA Cup finalists Crystal Palace left to play, Koeman's side are sat in seventh place, just two points behind West Ham United and three adrift of Manchester United in fifth.