I think this, in an odd way, is where Nelson's plan of running down his contract has backfired.
If Cardiff were looking for a back up centre half (3rd or 4th choice) then they are not going to pay big money for anyone, to sit on the bench. So they had to find a top quality player on a free, Nelson ticked that box.
So Nelson found himself signing for a club that didn't necessarily need him because he was free, if he was still in contract and a fee of £5m was required then the team signing him would have had the intention of playing him regularly.