Celtic boss Ronny Deila says keeping his best players will take priority over acquisitions in the transfer window.
Chelsea striker Fernando Torres is understood to be set for a return to Atletico Madrid, according to Sky sources.
The 21-year-old Ivorian spent last season on loan with Bastia's Corsican rivals Ajaccio, scoring eight goals in 23 games.
Tallo is Claude Makelele's first signing since the former Chelsea, Real Madrid and France midfielder took over as coach of the club. Makelele led his squad in their first pre-season training session on Thursday.