Donaldson signed professional terms until the end of this season last year, but has so far appeared just three times for City. The first of those was at Port Vale in the LDV Trophy, where he scored 12 minutes after coming on as a substitute.
He has been a regular scorer at reserve and junior level, and enjoyed three months on loan at Harrogate Town last year.
Scarborough - who have ex-Tigers Michael Price and Scott Kerr in their ranks - have won one, drawn one and lost one this season. Donaldson could make his Seadogs debut against Burton Albion at the McCain Stadium tomorrow.