Dier was one of several senior players left out of the Spurs squad along with Hugo Lloris, Djed Spence, Japhet Tanganga, Sergio Reguilon and Tanguy Ndombele.
Postecoglou has been honest about the need for Tottenham to offload players during the final weeks of the transfer window, but he suggested nothing should be read into Dier’s absence.
Reports on Sunday which linked Dier with a move to Saudi Arabia were later shut down and he trained at Hotspur Way earlier that day, but he may have to assess his playing options now he seems to be fifth choice at centre-back.
“Eric is part of this team. We left a few out,” Postecoglou said. “We left some players on the bench that are very good players. We need a strong squad, it’s not about 11 players.
“Eric is in the same boat as all the other boys. He’s working hard in training and available for selection.
“My decisions then are what I think will give us the best chance of success for any given game and then we reassess the week after. Nothing really unusual there.”
Dier was a regular under Postecoglou’s predecessor Antonio Conte and made 42 appearances last season.
The summer arrival of Micky van de Ven from Wolfsburg has pushed Dier down the pecking order and despite featuring in pre-season, Ben Davies and Davinson Sanchez appear to have also moved ahead of him.
Dier joined Spurs in 2014 but was snubbed for the captaincy roles with Son Heung-min named skipper and James Maddison and fellow centre-back Cristian Romero listed as vice-captains.
Romero was part of a new-look defence at Brentford with debuts handed to goalkeeper Guglielmo Vicario, Van de Ven and left-back Destiny Udogie.
Postecoglou added: “Obviously we had Micky, Destiny and Vic, three of our back five, making their debut for the clubs and anyone will tell you when it comes to the defensive side of the game, the more understanding you have, the better you are.
“We obviously took a bit of a gamble throwing them all in but I thought all three handled themselves really well. It is not an easy place to come, you get put under pressure with balls coming into the box and I thought they all handled themselves really well.
“Micky has only had three sessions with us so I could have waited to put him in there, but my feeling is he will be a very good footballer for us and the quicker we introduce him to Premier League football the better he will be.”
Sunday marked the start of the post-Harry Kane era for Tottenham, but his departure to Bayern Munich on Saturday night did not alter the plans of Brentford too much.
Bees boss Thomas Frank said: “No, that is the short answer. Of course he is a different type to Richarlison. He is the England No 9 compared to the Brazilian No 9.
“That we know and the only tweak would have been that if Kane drops down deep, we would have needed to be aware of his fantastic passing skills and get closer to him.
“Richarlison is more about the runs but our game plan is our game plan with the things we believe in.”
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