The former Celtic and Southampton shot-stopper has benefitted from Guglielmo Vicario's serious ankle injury to start each of Spurs' last seven games in all tournaments, and he has been largely solid in between the posts.
However, Forster was at fault for two of Man United's goals in Thursday's quarter-final, and he also did not cover himself in glory for Jonny Evans's last-gasp consolation in North London.
Forster's error-strewn display will no doubt add fuel to the fire surrounding a possible goalkeeping arrival in January, and speaking to Sports Mole, ex-Spurs man Kasey Keller can envisage Postecoglou pursuing a more youthful option.
"It must be a concern for any manager if your number one gets injured, but this is where if you are Fraser then you must take your chance," the former USA international said.
Postecoglou could "look to the future" after Forster mistakes
"He is an experienced goalkeeper, and if he can perform well, get clean sheets and impress then when Vicario is back fit, the manager will have a decision to make.
"I had this when I was at Spurs when I took over as number one from Neil Sullivan. But for Ange though, he may look to the future and recruit a young goalkeeper who is available."
Forster's first nightmare moment on Thursday evening came when Spurs had already gone three goals to the good, as the Englishman's lackadaisical pass was intercepted by Bruno Fernandes, who teed up Joshua Zirkzee for a tap-in.
The 36-year-old's evening then went from bad to worse when Amad Diallo charged down his attempted clearance and cut the deficit down to one, but his blushes were spared when Son Heung-min's corner flew over the head of Altay Bayindir and into the back of the Man United net.
Evans struck the seventh goal of a mad-cap quarter-final in added time, but Postecoglou's crop ultimately progressed to the semi-finals, where they will face holders Liverpool following the conclusion of Thursday's draw.
Which young goalkeepers could Tottenham target?
Tottenham are rumoured to have cast their eye to the Championship as they pursue a new goalkeeping option, with Burnley's James Trafford and Sunderland's Anthony Patterson both tipped to move to the Tottenham Hotspur Stadium.
Angers' Yahia Fofana is also alleged to be on the radar of the Lilywhites, who do have Brandon Austin and Alfie Whiteman in their ranks as well, but the two home-grown talents are yet to make their breakthroughs.
While Forster may very well move on from Tottenham at the end of the season when his contract expires, a January departure is highly unlikely, so Spurs may keep their war chest closed until the summer transfer window.
Despite Thursday's disaster-class, Forster's spot in goal will no doubt remain safe for Sunday's Premier League blockbuster with Liverpool, which comes just three weeks before they collide in the first leg of their EFL Cup semi-final.
Forster has kept eight clean sheets in 30 appearances for Tottenham, who signed him on a free transfer in 2022 when his contract at Southampton expired.
Kasey Keller was speaking to Sports Mole on behalf of William Hill Vegas.
Written by
Ben Knapton