Mauricio Pochettino felt that referee Anthony Taylor missed a “clear foul” on Dejan Lovren in the build-up to Tottenham’s first goal.
The Saints boss claimed his centre-half was pulled back by Nacer Chadli before Kyle Naughton crossed for Christian Eriksen to score.
It was a major turning point in the match, coming just two minutes after the visitors had taken a 2-0 lead.
It proved to be the first stepping stone on Spurs’ road to a dramatic comeback, which culminated with them winning the game 3-2 in added time.
Pochettino said: “Their first goal came from a clear foul to Dejan Lovren, so their goal should never have been allowed to stand.
“I’ve been told that it was a foul and, from my point of view on the pitch, I think it was a clear foul.
“Then the 2-2, with the goal by (Christian) Eriksen, came very early in the second half. I thought that completely changed the dynamic of the game.
“When you receive a heavy blow such as that one I think that changes completely the dynamic of the game, and that’s what happened.”
Saints had certainly started the game in superb fashion and looked to be heading towards a comfortable victory.
However, once Tottenham got back into the match, they soon began to dominate, and put Pochettino’s side under a lot of pressure during the second half.
Overall, though, the Saints boss felt it was cruel on his side to have come away with nothing from the match.
“I thought we had a very good game overall,” he said. “We were up 2-0, we were controlling it, especially in the first half.
“I thought we deserved more, but there were some refereeing decisions that did not go our way today.
“Despite that, I thought we did very well against a very good Tottenham side.”
Although Pochettino was frustrated with the officiating, he did concede that Saints were not without blame, with Nathaniel Clyne and then Lovren making errors that led to each of Tottenham’s first two goals.
“It’s clear that we made some mistakes that are actually quite difficult to accept that we made them.
“It’s clear that we need to improve, especially individually, and that’s part of football.
“We always need to, and want to, improve.”
Pochettino, whose side have conceded 11 goals in their last five games, added: “All managers are worried and concerned when you lose. Of course we’re disappointed and of course we’re worried about that and concerned about it.”