Brazil forward Richarlison has been diagnosed with mumps and will miss Thursday's Copa America quarter-final against Paraguay.
The Everton star started the hosts' first two group matches before sitting out the big win over Peru, and he will again be absent in the last eight.
Richarlison is being kept in isolation due to the contagious nature of the illness, with Brazil stating he needs "full rest and medication" in order to recover.
The rest of the Selecao squad will be vaccinated following the match with Paraguay in Porto Alegre.
"Confirming Allan before the game is to give him confidence," Tite told his pre-match news conference. "It is for him to know that the coach has no doubt.
"If it was another player, like Dani Alves, for example, or Thiago Silva, it would be different. But in Allan's case, he's going to play."