I am a huge fan of Poch, but...
He doesn't use the squad well, and he, imo, makes wrong tactical choices when he subs. Both are important reasons why our subs have little or No positive influensa, and why we become weaker post sub.
We line up with close to the same eleven every game. Even in the cups, we have mostly used our first team. This, mixed with some injuries to our rotation players, means they don't get the match readiness we need.
It's hard to make an impact when you get 3 minutes of first team football a week, like Onomah, 8 minutes like Chadli, or even 20 minutes like Son.
The latter struggles with his first touch, his combination play, his decision making and his defensive teamwork. All those factors will most likely improve with more game time, but are hard to improve in training or on the bench.
If Poch wants more impact from the bench, he needs to rotate more, to keep more players more involved.
Tactically I find Pochettino's choices to be strange at times. People are talking about us as potential title candidates. I do not agree. Part of the reason for this is that our nine draws aren't coincidental.
Previous champions have had a tendency to, in games where they are not in front in the last ten-fifteen minutes, completely take control over the game, dominate their opponents, and get the goal or goals they need.
What we seem to do is lose control towards the end, which imo is often because of the subs, and maybe partly due to players getting tried. It was said a couple of games ago that we would be league leaders if games only lasted one half.
By taking off Eriksen and putting on Son, we lose control. The latter moves more offensively, he is not as good at holding on to the ball, he is not as good at defending, neither positionally nor as a ball winner. The same goes for most of the subs we've used - partly, as mentioned, because of the lack of match readiness, partly because of skillset, mentality and how long they've been with us.
If Poch had brought on a Mason, a Bentaleb, we could've established more dominance and had more of a chance to win and less of a chance to lose.
We were very lucky not to lose against a ten man Watford after poor subs, and could've easily lost against Everton in the last fifteen as well.