N'Koudou's expected departure will be the first piece of transfer business that Tottenham have done involving a senior player this summer, having become the first Premier League club in transfer window history not to sign a single player.
Sky Sports News claim that negotiations are ongoing as Mainz would prefer a permanent deal, but the more likely outcome is a loan deal with a purchase option at the end of the season.
The Frenchman signed for Spurs from Marseille in 2016 but has only made nine Premier League appearances since then, playing just one minute of league football in the first half of last season.
He was loaned to Burnley in January but also struggled to get in the team at Turf Moor, starting only two matches for the Clarets.
Despite Tottenham's lack of summer signings, N'Koudou was not included in the squad that travelled to Newcastle at the weekend and would appear to be surplus to requirements.
For example, Tottenham don't need the money this summer because they haven't made any new signings, so delaying N'Koudou's permanent departure will free up some space in next year's budget.