The match will be played at Dragon Park, the Welsh National Football Development Centre in Newport, with both clubs currently on training camps in South Wales.
The Australian, who left Japanese side Yokohama F Marinos, said "it doesn't really bother me" when asked if he had concerns over being second choice.
Postecoglou admitted that the most important thing would be to bring success to Celtic, after their run of nine successive league titles was brought to an end by Rangers last season.
After Saturday, Celtic will make the short trip across the Severn Bridge to face Bristol City next week, before flying back to Glasgow to take on Preston North End ahead of the first leg of their Champions League second qualifying round tie against FC Midtjylland.
Charlton have ambitions of their own for the 2021-22 campaign, with the Addicks looking to win promotion back to the Championship.
The South East London club finished seventh in League One last season, only missing out on a playoff place on goal difference despite beating third tier winners Hull City on the final day.
Nigel Adkins was appointed manager in mid-March following Lee Bowyer's departure to Birmingham City and lost just one of the final 10 games of the campaign (W5 D4), while no League One side conceded fewer goals in that period.
It means that hopes are high at The Valley, particularly because Adkins already has two promotions to the Championship on his CV with Scunthorpe United and Southampton.
Also on Charlton's pre-season scheduled are fixtures against Dartford, Reading, Crystal Palace and Fulham before they begin the League One season against Sheffield Wednesday on August 7.
Chris Gunter was part of Wales's squad at Euro 2020 so will have been given a break following the tournament, meaning he is not available for this match.
Jake Forster-Caskey, who has recently signed a new contract with the Addicks, suffered an ACL injury in the penultimate game of last season against Lincoln City so is another who will not feature.
With several Celtic players not yet back in training after representing Scotland at Euro 2020, Ange Postecoglou gave opportunities to several youngsters against Sheffield Wednesday and should do the same here.
Charlton Athletic possible starting lineup:
MacGillivray; Matthews, Inniss, Famewo, Purrington; Jaiyesimi, Dobson, Morgan, Gilbey; Stockley, Washington
Celtic possible starting lineup:
Barkas; Ralston, Welsh, Murray, Urhoghide; Soro, Shaw, Connell; Moffat, Henderson; Ajeti
We say: Charlton Athletic 1-2 Celtic
While Celtic are still missing many of their key players, they should still have enough quality to beat Charlton, who are a similar level of opposition to the Sheffield Wednesday team the Bhoys got past on Wednesday. Postecoglou will want to grow confidence as much as possible going into the new season.