While Newcastle were on top in the second-half, Spurs did squander numerous chances to seal the win, with Kane hitting the post.
Those misses proved crucial, with Magpies substitute Joe Willock smashing in an equaliser with five minutes remaining to leave Mourinho cutting a frustrated figure.
Spurs have made a habit of giving up points from winning positions, as their title challenge faltered after a promising start to the season.
And the latest case of the Londoners dropping points to a team fighting to avoid relegation has Souness concerned.
“Spurs have given up 15 points from winning positions,” he told Sky Sports in his post-match analysis.
“That’s not a Jose trait. That’s worrying. If you want to be a top team you can’t give up leads the way they do.”
Mourinho shared Souness’ concern at Tottenham’s penchant for throwing away leads.
“Same coach, different players,” the Portuguese told BBC Radio Five Live when asked about Mourinho teams usually being good at keeping leads.
“I am frustrated. We needed to win the match and we came to win,” he added.
“We were not far from winning it but, at the same time, we created some instability on the game with so many individual defensive mistakes that we made.
“We always gave Newcastle a chance to be in the game. Of course, we had the biggest chance of the game to make it 3-1 and kill it, then we concede a game the next minute.
“There are mistakes which I probably shouldn’t even call mistakes because they are related to qualities that players have.”