Midfielder Morgan Schneiderlin and full-back Nathaniel Clyne were sold to United and Liverpool, respectively, before the start of this season, and Koeman said the transfers had hurt the Saints' chances of repeating last season's feat of finishing seventh and qualifying for a Europa League playoff spot.
"When I came to the club, I knew already that we would lose some good players, and we lost, after last season, Schneiderlin and Clyne, important players for the team," the manager was quoted as saying by the British media.
"Of course, we would like to keep the best players because that means we can grow and grow.
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"If not, we need to spend money, and we don't spend a lot of money."
Southampton, who play Liverpool in the quarter-finals of the League Cup later on Wednesday, occupy 10th place in the table and have lost three of their last four league games.
"After the last transfer window there was one team in the Premier League who got money out of the transfer window, and that was Southampton," Koeman added. "Nobody else. Everyone else spent more money than they got in.
"I'd like to change that but I'm realistic. I know the situation of the club.
"It'll be really difficult to reach the same level because in one-and-a-half years, we lost 10, 11 players -- a whole team.
"It is not always easy to sign new players and they need time to adapt. It's difficult. Maybe that is one of the reasons why the level is still not consistent at the moment."
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
