Spurs began the campaign with a 4-1 win over the Saints, with five of their six summer additions on the bench and Richarlison suspended.
Conte's side justified the pre-season optimism with a ruthless display, as goals from Ryan Sessegnon, Eric Dier, a Mohammed Salisu own goal and Dejan Kulusevski earned them a comeback win.
Ivan Perisic, Yves Bissouma and Clement Lenglet all came off the bench in the second half, while Djed Spence and Fraser Forster were unused substitutes. Asked about not starting his new players, Conte said they would have to earn their place in the team.
"I think I used common sense because in this moment we are starting the new season and the old players reached a big achievement last season to get a place in Champions League," the head coach said.
"In the last 10 games I think we played really well and we had fantastic results. My message was very clear that I trust a lot in the old players and in this moment they are more reliable than the new players because they need to continue to work with us and continue to implement football knowledge.
"With the old players, I am in a step forward. For this reason I prefer to count on the start on the older players but I think it was right after this game and the final result. At the same time I really enjoy and I am really happy because we have new signings that are going into our process and for sure they will help us to improve.
"But they have to know they have to fight and they have to give me the possibility to make rotations."
Sessegnon, Emerson Royal and Kulusevski all responded to fresh competition for places with excellent displays, with the English full-back also having a goal disallowed and earning a standing ovation when he was replaced by Perisic just after the hour.
"I’m really pleased for Sess because we are working very well with him, especially on the physical aspect because in the past this player had many injuries and after one or two months, then again injury, muscular problems," Conte said.
"He’s working well and we are giving him stability in the physical aspect. I think it’s very important that we signed a player like Ivan Perisic because he [Sessegnon] can see the way to learn something about Ivan. We are talking about a very important player in Ivan, a top player.
“Sometimes it’s good to bring into your team players even if they are a bit over-age. If they are a good player with experience, they can bring this experience to improve young players in your squad. I think Sessegnon is doing this.
“I was really pleased because today he played a good game. Emerson played a good game. Now we have to continue in this way.”
Sessegnon got behind Kyle Walker-Peters to head home Kulusevski's cross and level the scores after James Ward-Prowse's volley had given the visitors a surprise lead.
“If you remember maybe five minutes before there was a cross and he was behind his opponent - on the bench we were really upset with him," Conte joked. "Because he has to attack the second post and find the right position.
“After three minutes then he scored and we were pleased and celebrated a lot because five minutes before he wasn’t in the same place to score. Today he scored another goal. He was always in our offensive position. It was good. I’m happy for him because he has great potential but has to show this potential.”