It was not invalid. There was no walkover or withdrawal on the part of Bellucci, if he was your main pick. Only if you chose Harrison in that match would your backup be used, because Harrison withdrew.
Actually I do believe that any pick involving the Bellucci/Harrison match is invalid since this match did not happen.
This is not explicitly stated in the rules however, as it says "in the case of a withdrawal of your main pick..." and doesn't say anything about the withdrawal of your main picks opponent.
However rule 6 states you may only choose from matches posted on the OOP for that day. No OOP had Bellucci vs. Brands on it so by that rule I don't see how Bellucci can remain your pick.
I can't recall an instance where this has occured before so I'll wait for confirmation from another board member. I will PM them all to make sure.
If this match is invalid then hopefully we can eidt the part about an opponents withdraw into rule 4
It seems fair if you wanted to pick Bellucci to beat Brands as well but we don;t know if this is the case. It would be unfair to Bellucci pickers who had a correct Backup if Brands had won as clearly Brands/Bellucci was a much closer match than Bellucci/C.Harrison. Had I picked Bellucci(I almost did) and after the fact saw the withdrawal I would not have wanted Bellucci to be my pick as imo Brands had a very good chance at beating him.