The Argentine attacking midfielder was set for the Saints scrapheap in the summer after spending last season on loan at Hull City.
Koeman gave Ramirez his first taste of action in over a year for the south coast club in their 6-0 Capital One Cup at MK Dons midweek and has revealed there could be more game time for the former Bologna man.
"What's changed? What has changed is we passed the transfer window," the Dutchman said.
"We had some injuries with some players and I gave some players a rest.
"Still, he isn't the first choice on the position in front, but I know his qualities.
"He is technically a very good player and if he is physically on level, yeah, why not? Why not? It's competition.
"It's all about your performance during the whole week and if you get the opportunity to play show your qualities."