Hamilton will be moving to Ferrari, where he will displace current star Carlos Sainz, with a replacement yet to be decided for the champion at Mercedes.
The Spaniard was initially tipped to join the team, however their boss, Toto Wolff, set his sights elsewhere on the grid.
The Italian was catapulted from the Formula Regional Championship into F2 this year, where he currently sits seventh in the drivers’ championship.
Despite this, Mercedes bosses remain confident in his talent, with Antonelli participating in a series of tests for the team, alongside claiming two wins in F2.
According to reports, Antonelli is expected to be announced as Hamilton's replacement by Mercedes ahead of the Italian Grand Prix, however Horner has revealed that this could be a ‘risk’ for Mercedes.
“I’m sure whatever options they choose to take, they’ve thought carefully about it,” Horner said to Sky Sports F1.
“And if they choose to give Antonelli the seat then you’ve got to commend them for taking a risk with a youngster.”