Yes
First of all, he was already skinny. There was nothing to really lose and much more to gain
Then depends on his body type and genetics
Then after all that, this shyt was his job. All he had was time to train. His boxing might’ve not been as sharp as a real pro during the first Creed, but if you put in enough time, work, and recovery into the body then its possible especially if it’s just the looks. Then the money behind him getting the looks…safely. This not the 80’s.
Especially if thats all you had to do. This is Hollywood so maybe, but he was young enough during the first Creed to not need the help if he had the discipline