Allan's impressive form has been cut short by a hamstring injury.
The Brazilian midfielder had picked up where he left off against Chelsea with an all-action display against Leicester City this evening.
But in the 39th minute at the King Power, the 29-year-old pulled up, holding his right hamstring, as he chased back as the hosts hit Everton on the counter-attack.
Allan was stretchered off and replaced by Andre Gomes with the Blues 1-0 ahead.
Everton have lost Seamus Coleman for six league games this season due to a persistent hamstring issue while James Rodriguez is still a doubt for this weekend's game with Arsenal.
Allan now looks set to miss that game and, most likely, the visit of Manchester United in a week's time.
Depending on the severity of the injury, the summer signing could also miss the upcoming games with Sheffield United and Manchester City, before the month is out.