The defender has joined on a one-year deal following his release by Liverpool.
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 Algerian international looks set to leave Spurs this summer.
Arkadiusz Milik is prolific for his club and West Ham United should consider a move for the Ajax forward.
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.