Tottenham's defense was really weak in the few matches they played, Conte's men were unable to keep up with City's game after the second half, implementing a three-back formation made it difficult for Tottenham's defense to stop Manchester City's attacks. A two-goal advantage in the first half made Tottenham unable to maintain victory, because Pep Guardiola's men began to freely create opportunities and managed to score four goals in the second half.
Defense is a big problem for Conte's men, because so far they have conceded 31 goals until the 20th week, Conte must really solve this problem, if Tottenham's management continues to entrust him as a coach, because of rumors circulating about Konte's future at Tottenham very questionable.
It's not a tactical issue, it's about speed and that's what the other teams have these days. In todays modern world, speed is the killer, all the best players in the world are fast, look at arguably the best player right now, Mbappe, known for his speed. So, Tottenham defense needs to be a lot faster, or they need to just lay back a lot which would help them out for not being as fast.
I know that would be a tactical solution, but they do not need a tactical solution if they were fast enough. Long gone the days of puyol, pique, maldini type of technically gifted defenders, you would need people that are super fast on defense to be able to catch up with those amazingly fast strikers and wingers.