I agree, but I don't think Zlatan is the perfect fit for our offense. Not because of his lack of mobility, but because he tends to hold on to the ball far too long in my opinion and poor conversion rate. With young players you have to make it easy and it's harder to execute when you hold on to the ball and let the opponent get into their defensive shape. Still miles ahead of any other option we have as shown by the last few games.
Actually, someone holding the ball is exactly what we need when winning the ball back and transitioning. You are not suppose to counter as soon as you win the ball, good teams rarely do this because they have good enough techinque/passing and hold up to retain possession after winning the ball. This is difficult to do for average/bad teams hence they mostly can't get out of their half against top teams. This is one of the reasons why alot of smaller clubs play with a big man upfront, because they don't have the skill to play themselves out of their half like bigger teams do.
When Zlatan played for Barcelona, he wasn't used as a hold up player since they were so good at passing it around and retaining possession without having to use him. Here, Ibra and Pogba are the best ones at holding it up and you can clearly see how much we're missing this since we don't have such quality players to do it differently.
The argument for Zlatan holding us back(apart from conversion) is the lack of pace mostly. The problem with this statement is that we haven't got the fast fluid movement upfront since he got injured. Yes we haven't been able to always play our best team but that shouldn't be an excuse. Zlatan was not playing with the best possible 11 around him all the time either. Rooney, Martial and Fellaini in the beggining at the season for example didn't stop him from playing well.
After Zlatans injury we were still playing the same players that were part of the first team at some point during the season. Against Tottenham, we were arguably only Pogba away from what shouldn't be so far away from full strength with the exception of Tuanzebe. It's not like our pacey players upfront had to play with a bunch of youngsters behind them.
Jose reverting to negative tactics after Zlatan's injury is probably directly because of loosing Zlatan's hold up abilty, and Pogba's as well in some games where he didn't feature. Not only that but loosing our only goalscoring outlet probably had something to do with it. If you're less likely to score then the apparent option is to try minimising your opponents chances, at least it is with Jose's tactics.
I understand that Rashford/Martial haven't had enough chances to prove what they can do as a lone striker but this far everything points to us having to get someone to replace Zlatan for next season. Although I think Martial is more suited to the wing anyway. There isn't really much time left for anyone of the attackers to cement a place for the new season. Even if we get Griezmann we still need another striker. I'm pretty sure Lingard, Rashford and Mikhi will stay but Martial is doubtfull I would say for next season.
Edit: Sorry for the long post.