Whether or not it is not necessary to be ordained is subjective and is dependent on a person's circumstances, aptitude and level of practice or attainment if any. Generally I would imagine one who has become enlightened would probably have had many life times of practice and at some point, one would have been ordained.
If one is at the earlier stages of the path, in order to gain progress faster, it would be necessary to be ordained and join an institution of Buddhist learning and practice such a monastery. Being an ordained sangha in monastery would mean full time practice and minimal distraction from the worldly/mundane world. Just like an athlete training for the olympics needs full time training in order to be qualified, what more one who is training to become enlightened, a goal a million times higher in which case one may not even attained it in one life time.
However, if one if a high level Bodhisattva, whether or not one is ordained does not make a difference because the level of attainment would be such that one will not be affected by the distractions of the worldly world. One would already have very strong imprints of practice from previous lives. One would thrive even in the midst of the most samsaric environment and be able to transform any obstacles and difficulties into the path of practice. Enlightenment is just a matter of time.
In conclusion, I would say ordination would be very necessary for one at the earlier stages of the path in order that one's spiritual foundation is built for future enlightenment.