Ronald Koeman believes persuading Morgan Schneiderlin to stay at St Mary's was Southampton's biggest transfer-window coup.
Koeman has tipped new recruits Sadio Mane and Toby Alderweireld to thrive on the south coast, but admitted holding onto Schneiderlin was Saints' most important move.
Mauricio Pochettino fought hard for France midfielder Schneiderlin to follow him from Southampton to Tottenham, only for Saints to rebuff a number of bids.
Schneiderlin initially vented frustrations at being denied a transfer, but new manager Koeman has now claimed the 24-year-old decided to knuckle down on the south coast of his own accord.
"I hoped, and in the last three weeks I would say yes, him staying has been the best part (of the transfer window) for us," said Koeman.
"He's a fantastic player and an important player for the team.
"That gives the rest of the lads the confidence and ambition that we spoke about.
"It was not an easy one because he was disappointed in the beginning, but I think that was a normal reaction.
"But after that his behaviour has been professional and the one answer he gave was on the pitch, and that's always the best answer that you can give.
"It's important for us to keep that kind of player in the team.
"I spoke several times with Morgan, but finally the player by himself changed his mind and that's the most important thing."
Koeman holding onto Schneiderlin stemmed a summer-long Southampton talent drain, but the former Dutch defender admitted it is too soon to make predictions on whether the club can fend off suitors again in January.
"I don't know about that, we will just have to wait and see in January," he said.
"I'm not thinking about January at the moment, I'm just preparing our team for Saturday's Newcastle match.
"That's the most important thing at this moment."