Toby Alderweireld says Ronald Koeman was key to his decision to join Southampton.
The Atletico Madrid defender is on a season-long loan.
"The most important thing was the coach," said Alderweireld.
"He called me and said 'We really want you' and, for me, for one year, that was the most important thing, that I get the confidence from him and to play football.
"He called me when I was in Madrid. We had a good conversation, and he said 'Yeah, no problem, if you want to join us we will be taking you', so for me it was very good. Footballers need confidence. That's the most important thing - and that's what he gave me."
Alderweireld added: "Of course, some people asked me before: 'Is Southampton the right decision?' Now they say 'Yeah, good decision!'"
