Spurs will not receive a fee for the move, with the priority instead being further game time to help decide whether Donley has a long-term future under Roberto De Zerbi.
Donley travelled with the first team on their pre-season tour of Australia and Asia and is said to have impressed the head coach, but the arrivals of Sandro Tonali and Mateus Fernandes this summer have left little room for him in midfield this season.
He spent the first half of last season on loan at Stoke City without securing consistent minutes, before a recall and a switch to Oxford United, where he impressed even as the club were relegated to League One.
Tottenham are also negotiating a possible permanent departure for Pape Matar Sarr, who has yet to feature this season and is attracting interest from Aston Villa. De Zerbi has said the Senegal international is dealing with a hamstring issue, having been given extended leave following the World Cup.
Spurs have already raised more than £100m in player sales this summer and are believed to be seeking around £20m for the 22-year-old, whose exit would continue a significant overhaul of the club's midfield options this window.
Both situations reflect the scale of Tottenham's midfield rebuild since De Zerbi's appointment. Beyond Tonali and Fernandes, the club have also strengthened at the top end of the pitch this summer with Savio and Omar Marmoush, funded in part by a series of departures that have already brought in nine figures.
Donley's loan and any eventual Sarr sale would free up further squad space and wages as Tottenham continue to reshape the group before Tuesday's transfer deadline.