You would see them very obviously itchy and hassled by it, if it were scab and it would be unlikely to be only one that had it. If you watched them for a ?hile, you'd see them scratching and biting themselves.
How big is the patch? Can you get a photo?
Many other things can cause bald bits. One of Dad's new Ryelands had a bald, sore patch which seemed to be combination of too enthusiastic shearing combined with rainrot. We treated for the latter (antibiotic injection and Dermasol on the sore bit) and she's now growing back her wool.
Equally it could be a bit recovering from flystrike.
Many of us treat for scab by injecting with Dectomax around this time of year, which kills biting, sucking things and which also treats roundworms.
Lol, too slow posting and both suggestions already given too 