Ramirez has struggled for appearances for the Saints ever since Koeman took charge, and was loaned out to Hull City last season.
However, the Uruguayan international has come back into Southampton's first-team this term, and Koeman says he has been impressed by the attacking midfielder's professionalism.
"I was always positive about his individual qualities as a player,” the Dutch manager said.
“Sometimes I had my doubts about his physically in the Premier League, because a lot of time injuries, he was on loan to Hull City last year, because it was difficult.
“What I like from Gaston is with all that kind of problems, with all that not a lot of opportunities, he was always working hard, he was always a very good professional.
“That’s important and then I am the first to give him that chance to play, that opportunity, if he is ok."
Ramirez played a big part for Southampton in their latest fixture against Liverpool, as he came off the bench to set-up their equaliser.
The 24-year-old got his head on a set-piece, and by doing so he set-up Sadio Mane to tap home from close-range and help the south coast side to take a point from Anfield.
Ramirez's performance was a very encouraging one, and there are suggestions that he could come back into contention for a starting berth now.
That will be music to Ramirez's ears, as he has played just two Premier League matches in as many seasons for the Saints, since Koeman took charge.