The French midfielder is set to complete a move from Rennes.
The midfielder arrived in Hertfordshire for a medical this week with the club looking set to complete an £8million deal with Rennes, a club-record for the Hornets.
They had been linked with a move for him in the summer and were close to a deal but that move never came to fruition, as Rennes rejected their offer.
It seems they had no plans to sign him in this transfer window either, with a move potentially on the cards next summer instead.
That was until Liverpool declared their interest in the player last week and forced the Hornets’ to make their move, as reported by the Mirror live transfer feed.
The midfielder is seen as a long-term prospect by Quique Sanchez Flores’ side and they will be keen to see him gain continued first-team football at Rennes for the rest of the campaign.