Ramirez was the Saints' record transfer when they signed him from Italian side Bologna for a reported 12 million pounds ($18.3 million) in 2012, but his time in England has been plagued by injuries and questions about his attitude.
The Uruguayan international spent last season on loan at Hull City, and was widely linked with a move away from the Saints during the summer, but a combination of factors appears to have helped him force his way back into Koeman's thinking.
"What has changed is the transfer window has gone. We had some injuries with some players and I gave some players a rest," Koeman was quoted by the club website (www.saintsfc.co.uk).
"Gaston is part of the team, he is training well, his behaviour is positive. I'm the first to give that kind of player a chance when we need somebody," the manager added.
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Koeman also said Graziano Pelle should be available for the game against Swansea despite missing the MK Dons trip due to a foot injury.
Meanwhile, Swansea manager Garry Monk has called on his players for a strong reaction after going three games without a win.
The Swans were dumped out of the League Cup by Hull City in midweek after losing away at Watford in the league a fortnight ago, but played out a hard-fought goalless draw at home against Everton last weekend.
"We always react well when we get setbacks," Monk said. "We did that well against Everton last weekend and I think it was a really good point in the end and a fair result.
"The effort and commitment is there and we're creating lots of good chances -- we were the better side again in midweek against Hull and should have won.
"We've had good performances apart from at Watford and I want us to continue with the same attitude and determination that we're showing right now," he added.
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
