Apologies in advance for taking the bait.
For all of his talk about what we did with the ball, it never felt like we had a clear plan in possession under Martinez. We used to pass into pressing traps pretty regularly and we conceded too many goals off of cheap giveaways as result. I also think that, just as with Koeman, Lukaku helped mask genuine chance creation issues.
Teams that effectively prioritize possession usually close the opposition down directly after they lose the ball for a few seconds before falling back into their defensive shape. We rarely ever pressed like this under Martinez, probably partly down to poor fitness, and we didn't have a good defensive shape either. That's a recipe for defensive disaster, whoever the players.
Our fitness wasn't good enough under Martinez. This showed up in two ways. (1) We weren't fit enough to apply pressure to our opponents which, as said previously, is important for a team that wants to play on the front foot. (2) Even when one of Martinez's plans worked to perfection, and he did have some good one-off plans, we were always a threat to fade around the 60th minute and throw the match away.
Martinez looked at the conversion rate from set pieces and wrote them off as an unimportant part of the game, choosing instead to focus on moving the ball (which, again, we really weren't all that good at). Set pieces are pretty well established now as a huge edge for clubs to prioritize them (https://statsbomb.com/2017/02/changing-how-the-world-thinks-about-set-pieces/
), not to mention that we left ourselves susceptible to repeated punishment from corners, indirect free kicks, etc. by not working on them enough.
We're still terrible at transfers now but that doesn't excuse what Martinez/Reeves did. They wasted a lot of money on players Martinez knew (Kone and Alcaraz were never good enough, Robles needed a lot of work to look remotely competent) and had a tendency to fall in love with players that had massive flaws. Besic was signed on the back of a few good World Cup performances in midfield (Martinez says we had followed him for longer but I'm not buying it. What he would have seen in that time was Besic playing in defense.) We could have signed Jonny Evans, who would have fit our passing approach and been an experienced partner for Stones, but instead chose to risk that money on Funes Mori, who isn't as good on the ball and is prone to massive lapses in concentration. Niasse has his virtues (we've seen that this season) but he was never a fit for a possession team and he was never worth the money. And that's not to mention other hopeful punts like Tarashaj (where is he?) and Rodriguez (may as well have burned the £600k). I'll give him Lukaku, Barry, Deulofeu, McCarthy (I don't think many saw him flatlining the way he has), and Holgate (paying off now) but there's not a lot of good in there.
Even if you think the recruitment would have been fixed with the money we have now (I'm not buying it, but okay), poor fitness, poor organization, and a lack of attention to detail still would have let us down. Martinez wasn't turning it around.