Tottenham revealed on Wednesday that Kulusevski had undergone surgery on an injury to his right patella he sustained in Sunday's home defeat to Crystal Palace, although the club did not give a timescale on his return.
But Postecoglou said at his pre-match press conference ahead of Friday's Premier League game at Aston Villa, live on Sky Sports, that the Sweden international will miss next Wednesday's Europa League final against Manchester United in Bilbao.
He said: "Disappointing news. Initially, we thought it wasn't too serious. It seemed pretty good but it kind of blew up the day after.
"He has had surgery and it will put him out for a few months.
"We will see [back for next season]. To be honest, I have basic information on recovery. It will put him out for a while."
He added: "No, it wouldn't be anywhere near 12 months, but again me putting a timeframe on wouldn't be right because he only had the surgery yesterday.
"It's fair to say months, but how many months I wouldn't speculate at the moment.
"I think he's grown as a player this year, he captained his country and then sort of got hit with a foot injury which set him back but then - typical him - he worked hard at rehab and was really important for us away at Bodo.
"Started looking like he was getting back to himself, which would have been great for us, so for him to get another setback is is disappointing, but mentally he's a very strong guy and I'm sure he'll use this as a catalyst to come back stronger."
Kulusevski's injury is a major blow for Postecoglou, with the Australian facing a midfield selection crisis ahead of the final in Spain.
James Maddison and Lucas Bergvall have both suffered season-ending injuries.
Maddison suffered a knee injury in the 3-1 Europa League semi-final first leg win over Bodo/Glimt at the start of the month, while Bergvall suffered ankle ligament damage in training ahead of the same game.
Speculation was rife on social media on Wednesday night that the teenager could make a shock return against Man Utd, but Postecoglou has quashed those rumours.
He said of Bergvall: "No, it's still the same sort of timeframe.
"Never long term, but he's just got out of his boot and taking his first steps on the grass. He'll likely be a while."
The absence of the trio leaves Spurs desperately short of creativity for next Wednesday's final with Manchester United in Bilbao, where they will aim to end a 17-year trophy drought.
Kulusevski has 10 goals and 11 assists in 50 appearances for the club this season.
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Postecoglou Confirms Kulusevski Ruled Out of Europa League Final After Knee Surgery
Tottenham Hotspur have been dealt a significant blow ahead of next week’s UEFA Europa League final, with manager Ange Postecoglou confirming that Dejan Kulusevski will miss the clash against Manchester United in Bilbao due to a knee injury.
The Swedish international underwent surgery this week after sustaining an injury to his right patella during Sunday’s Premier League defeat to Crystal Palace. While the club initially withheld a timeline for recovery, Postecoglou revealed during Thursday's press conference that the midfielder faces “a few months” on the sidelines.
“Disappointing news,” Postecoglou said. “Initially, we didn’t think it was too serious. It seemed pretty good, but the next day it swelled up and after further assessments, surgery was the best option.”
Kulusevski's absence adds to a growing injury crisis for Spurs, with midfielders James Maddison and Lucas Bergvall already ruled out for the remainder of the season. Maddison suffered a knee injury in the first leg of the semi-final against Bodo/Glimt, while 18-year-old Bergvall sustained ankle ligament damage in training prior to that fixture.
Rumours had circulated online that Bergvall could make a surprise return for the final, but Postecoglou dismissed the speculation.
“No, it’s still the same timeframe,” he said. “He’s just come out of the boot and started walking on the grass again. He’s progressing but still a way off.”
Kulusevski had been enjoying a strong season, registering 10 goals and 11 assists in 50 appearances across all competitions. His versatility and creativity have made him a key component of Postecoglou’s system, and his absence will be keenly felt as Spurs seek to lift their first major trophy since 2008.
“Mentally, he’s a very strong guy,” the manager added. “He captained his country earlier this season, came back from a foot injury and was starting to look like his old self. This setback is tough, but knowing him, he’ll use it as motivation to come back stronger.”
With the Europa League final fast approaching and creative options limited, Postecoglou will be forced to reshuffle his midfield plans as Tottenham aim to end their 17-year wait for silverware on the European stage.
Nottingham Forest manager Nuno Espirito Santo provided an encouraging update on Taiwo Awoniyi's condition after the striker underwent emergency surgery following an abdominal injury sustained during Sunday's 2-2 draw with Leicester City.
Tottenham captain Son Heung-min has lodged a criminal complaint with South Korean authorities after reportedly being targeted in an alleged blackmail scheme involving a false pregnancy claim.
Chelsea star Cole Palmer has heaped praise on Barcelona wonderkid Lamine Yamal, boldly declaring the 17-year-old as the best player in the world following his standout Champions League campaign.
Nottingham Forest defender Ola Aina believes the injury suffered by teammate Taiwo Awoniyi during their 2-2 draw against Leicester City could have been avoided if the offside flag had been raised earlier.
