In the ring there are rules.
On the street there are not.
To be good in the ring, and have a career, you must obey the rules.
The means knowing what the rules are.
If you know what the rules are, you can break them at any time.
So, if I'm a professional fighter who trains X hours a day, with lighting quick reflexes, bombs for punches, and never ending stamina...
...and I get in a street fight....
am I going to "obey the rules" or go for the most crippling move I know (that I'm not allowed to do in the ring)?
I submit to you that most fighters would choose option B.
Now, also, take into consideration the fighter's opponent "on the street".
If he's not a professional fighter, no experience...
...what chance does he have against a better conditioned athlete with the strength, reflexes, honed accuracy who does nothing but train for one on one combat?
I would say that probability favors the fighter, but there is always a puncher's chance.