Everton beat Barnsley 3-0 in a pre-season friendly on Saturday afternoon.
The former Manchester United man is worried their old rivals could be on the way back under the German.
The Celtic boss says he has been impressed by the 20-year-old's understanding of the game.
Leeds United lost 2-1 at Peterborough United on Saturday afternoon.
Newcastle United beat Lokeren 4-0 on Saturday afternoon.
The Tottenham Hotspur teenager needs chances to shine in the first-team squad.
The defender has opted to reunite with Brendan Rodgers at the Scottish champions.
Sam Allardyce chooses the best XI he has worked with, including Bolton Wanderers and Newcastle United stars.
The Championship season kicks off in just under two weeks but speculation persists regarding Jackett's future.
The striker left the Scottish Premiership side earlier this summer.