The Uruguay international, who spent last season on loan at Hull City, was heavily linked with a move away from the Saints during the summer as his time in England has been plagued by injuries and questions about his attitude.
"I was always positive about his individual qualities as a player," Koeman told the club's website (www.saintsfc.co.uk).
"Sometimes I had my doubts about his physical (ability) in the Premier League because a lot of time he had injuries -- he was on loan to Hull City last year because it was difficult.
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"What I like from Gaston is, with all that kind of problems and with 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 okay."
Koeman said he felt Aston Villa were too hasty when they sacked Tim Sherwood on Sunday after just 10 league games.
"Most of the time to change the manager is not better for the team or better for the club," Koeman said. "All the managers know, it's a difficult job, a difficult function in the club, a lot of pressure.
"You need understanding, you need support and to do this kind of decision after 10 games in my opinion is too fast."
The 52-year-old Dutchman admitted he doesn't know what to expect from Villa as a change in manager might prompt a different approach.
"It's a different competition and everybody knows we take it seriously," Koeman said. "Normally (a change of manager) gives always a reaction to the team.
"Players who aren't involved in the last few weeks maybe will get a chance under the new manager. That's possible, but we know every game is difficult."
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Sudipto Ganguly)
