Question - was it a 45K lost cluster, or a bad sector - they are different. Usually bad sectors are given an address, not a size.
Lost clusters are not usually a serious long-term problem.
Even if it was a bad sector, it is difficult to say whether this will rapidly get worse, or just an isolated event from a power surge or physical shock / movement.
If you are otherwise happy with your disk, I would monitor it carefully for the next few weeks before rushing out to buy. Keep your backups current.