The Uruguay international, 24, was a second-half substitute in the 6-0 League Cup win at MK Dons on Wednesday.
Ramirez, who spent last season on loan at Hull City, was Saints' club record signing when he cost them £12m in 2012.
"He's part of the squad, training well, well disciplined and not complaining. I admire that," said Koeman.
Ramirez's last appearance for the Saints was in a 3-1 victory at West Ham in August 2014 but injury and questions over his attitude have plagued his three years at the club.
Southampton also welcomed back left-back Ryan Bertrand for his first game of the season after undergoing knee surgery during the summer.
Koeman added: "He's good because he played 45 minutes [in an under-21 game against Reading] on Monday and another 35 minutes tonight. He is still maybe not ready for a whole game, but he's close to being back."
Southampton manager Ronald Koeman says Gaston Ramirez is back in his plans after the forward made his first Saints appearance in nearly 13 months.