There are two hitches to the treatment of bipolar disorder...

First - The struggle is keeping them on their medication. If treated correctly the medication works so well that the person starts to believe that they no longer need the medication and quite often just stops taking it, which leads to an ugly relapse.

Secondly - Besides what ever medication they are put on to correct the chemical imbalance that causes the disorder, they are also put on anti-psychotic mood stabilizers. Mood stabilizers take the peaks and valleys away. Manziel is a gunslinger and played the way he did because he was on a high, the mood stabilizers would prevent that euphoric high...

I have lived and dealt with some that suffers from bipolar disorder first hand. When the doctor was asked "when will my parent be normal again"? The reply was "we do not know what her normal is yet"...

Medicated Manziel will not play like manic Manziel.

I agree with number 1 completely. I think givin the fact that he will be surrounded by team doctors means he will be monitored daily and it would become part of his daily regiment.
As for number 2 I don’t think his play was ever erratic or inconsistent, just his behavior. This tells me he has the skills to play at a high level medicated or not.
He’s wasnt going out there winning on pure emotion or instinct. He has smarts , a good understanding of the game, and other skill sets so I think he would be just fine. It would be different if he had moments when he were stellar and others that were awesful but I don’t think he had that problem.
Either way his problem was behavior while a symptom of bipolar needed to be addressed with or without medication. While medicine may help in that regard he’s still going to have to change his behavior.

I didnt say anything about his play being erratic and or inconsistent. Because his high-highs will be knocked down the euphoric feeling of being 'in the zone' wont be the same.