On his form, Pochettino said: "Two words". He had made 24 appearances in all competitions as Spurs made a positive start to the season. That contract was due to expire in the summer of 2019.
Tottenham and Alderweireld had been negotiating an extension to the 29-year-old's current deal but those negotiations broke down.
Tottenham head in to the weekend action sitting second in the Premier League table after scoring 11 goals in their last two games so Pochettino will be looking for a win to reduce Liverpool's gap to three points ahead of their clash with Arsenal.
"Anfield was wonderful, every game Liverpool play at Anfield is a party and that sometimes makes a difference", he said.
"To play in your own stadium is the best thing that can happen in your life", he added. "I told you after the match in Barcelona that we really believe in him and he has incredible quality and only needs time to mature and show his quality. That increases the level, the competitive level in the squad and for sure helps to get good results".
Confirming Spurs may need to offload players next month, Pochettino added: "We're always open (to signing players in January), but it's going to be hard - maybe not impossible but hard".
With the January transfer window around the corner there will be plenty of focus on whether Spurs make any signings, given they did not do so in the summer.
"Maybe every day we talk (with chairman Daniel Levy and chief scout Steve Hitchen) but it depends on if players are going to leave and to have space to sign players".
"We really believe in him and we're sure that he's going to be an important player in the future for Tottenham".