You want to special summon a monster from your graveyard by using monster reborn BUT the card says it can only be special summoned by discarding a card from your handdo you need monster reborn?

A monster's special summon text only applies while it is in your hand. If a monster says it can 'only be' special summoned by a certain method, then assuming you do summon it properly, and it is then destroyed, then you are allowed to use Monster Reborn and do not have to pay any additional costs.

If the above monster was simply discarded from hand by something else, then it can not be special summoned by Monster Reborn. If it says 'cannot be special summoned except by..' then it can't be brought back with Monster Reborn even if it was properly special summoned first.