Sanchez joined the reigning Turkish champions on Monday night in a permanent deal worth just under £12.8million, with Spurs team-mate Tanguy Ndombele also heading to Istanbul on a season-long loan that includes a purchase option for next summer also worth £12.8m.
Both signed in time to be included in Galatasaray’s squad for the Champions League group stage.
Sanchez began the season as Spurs' third-choice centre-back under Postecoglou and came off the bench in the opening-day draw at Brentford before starting and missing a shootout penalty in the Carabao Cup second-round defeat by Fulham.
With Eric Dier having not made a single matchday squad yet under the Australian, summer signing Phillips was on the bench against Burnley at the weekend and could now act as centre-back cover, alongside Ben Davies, until at least January.
Phillips, 18, joined Spurs in July for £3m from Blackburn, where he played eight times in the Championship last season, and has been training full time with the first team, despite also appearing at Under-21 level.
The England U19 international is highly-rated but could now be thrust in at right centre-back if there is an injury to Cristian Romero, unless Postecoglou brings Dier back in from the cold.