It isn't a deal breaker - they may not have all the images they would usually collect but can make a reasoned assessment of his staging by his biopsy result (each core will give some indication of how close to the edge it has crept or whether it is still well contained), his DRE, his PSA score and if they believe there may be some spread, a different kind of scan.
Scans are not perfect ... my husband's said his prostate was completely clear when in fact it was already in every segment of the gland and had moved into the bladder so it was a good job our surgeon relied on his experience as much as the clever machines!!!