His Ultimate is the only reason why he is even playable right now.
Atrox Sucks because he only gets effective or reliable mobility from his Ultimate.
If your loosing lane, to renekton because hes E W Q E comboing you its not because your ultimate isn't giving you an increase in health the same thing would continue to happen if it did and you would still loose due to the fact that Atrox's Q is useless in that situation It will get canceled if you try to peel from that lane bullying and it has a ridiculous cool down timer and costs your hp.
You will be left with your E ability that is also pretty usless in that scenario as it is gonna cost your hp to deal ability power dmg and have a very weak slow effect that will not help you. He would be better off if his E was a shield or some kind of utility abilitie.
For instance if you gave Atrox Renektons Ultimate... Atrox would be completely unable to play with his kit. Having no mobility and being kited even harder than he is now. You would have very few chances to get a perfect Q ultimate off in a team fight and then immediately be CC'd you'd maybe get 2 autos off and then be transformed from Atrox into a Slothtrox, die, passive, Slothtrox.