Could have been better I am sorry to say.
I think the final match was too short, they ("they" being the writers of the script) rushed Lang getting "tired". Lang got too tired after only 2 rounds, even with over exerting and not pacing himself which was how most of Mohammid Ali's opponents lost it should not have happened THAT FAST. The movie would have been better if Lang had lasted a few more rounds maybe to round 5 or 6 before the tiredness became obvious.
I DO agree that Lang should not have gone the distance (or even most of the distance) for one thing Rocky really needs to have evolved beyond Lang to be believable as someone who could beat Ivan Drago in the sequel. Drago was so much tougher than Lang that if Rocky could barely beat Lang he would realistically stand no chance against Ivan. That is why I think Lang should have lost in round five or six, not round three.