Sporting Lisbon full-back Pedro Porro is in London for a medical before a move to Tottenham.
The Spain international, who can also play as a wing-back, will join on loan, but the deal includes an obligation to buy for 45m euros (£40m) this summer.
The 23-year-old, linked with Spurs throughout the transfer window, was previously at Manchester City but did not play for the first team.
Porro joined Sporting from City for £7m last summer after two years on loan.
Although the structure of the deal is complicated, it is anticipated it will be completed before the 23:00 GMT deadline.
Porro would become Tottenham’s second January signing after midfielder Arnaut Danjuma joined on loan from Villarreal until the end of the season.
Matt Doherty and Emerson Royal have been regulars on the right side for Spurs this season, but the Irish international is set to join Atletico Madrid on loan.
Djed Spence, who joined Spurs from Middlesbrough last summer for a deal rising to £20m, has moved to French side Rennes on loan for the rest of the season.
The England Under-21 international made his debut against Nottingham Forest in August but has only played six times.