According to 'Marca', the fee for the left-back will be around five million euros. Junior Alonso did not feature in Lille's opening Ligue 1 fixture and watched on from the stands as his side were 3-1 victors against Rennes.
The defender played 28 games last season for Lille, scoring twice. Alonso would be Celta Vigo's seventh signing of the summer after Juncá, Okay Yokuslu, Araujo, Fran Beltran, Jensen and Boufal.
Junior Alonso will come into the Celta frame as the Spanish side are lacking on the left side after the injury of Juncá.
Celta would also be Alonso's third club in his career. Born in Asunción, he started his career with Cerro Porteño and after four seasons moved to Lille for the 2016/7 season.