Trent Alexander-Arnold suffered a worrying twisted ankle in Liverpool's 3-2 defeat Arsenal that caused him "too much pain" to carry on, Jurgen Klopp said.
Manager Klopp said Alexander-Arnold's injury showed troubling signs of swelling, with Liverpool planning to get the right-back checked out on Monday.
The substitution of the England defender at half-time came after a dismal first-half performance, where he was arguably at fault to some extent for both of Arsenal's goals prior to the break.
For their first, after just 58 seconds, Gabriel Martinelli ran in behind the full-back to drive a low shot past Alisson.
Darwin Nunez brought Liverpool level, but Arsenal regained the lead just moments before the break when Martinelli again charged down the left, this time tracked by Jordan Henderson. Alexander-Arnold rushed to help, but his presence only appeared to confuse the situation and Martinelli was able to centre for Bukayo Saka to touch home from close range.
Klopp stressed Alexander-Arnold's withdrawal was enforced, however, just as Luis Diaz's departure from the game with a knee complaint had been late in the first half.
"Lucho [Diaz] we had to change before [half-time]," said Klopp. "Something with the knee, not good. He will have a scan and then we will know more.
"I have no idea about the extent, and then on top of that Trent is bad as well with the ankle.
"[He] twisted the ankle, not good. Trent never, in seven years, went off if he could have played on. He was in too much pain, it started swelling immediately, so we will have to see."
That injury looked to be caused when Alexander-Arnold was fouled by Martinelli.
Alexander-Arnold's England place has recently been called into question ahead of the World Cup, with the Liverpool man left out of Gareth Southgate's matchday squad for last month's Nations League game against Germany.
Man United could be without Toone until new year, says Skinner
Marc Skinner revealed that Manchester United could be without midfielder Ella Toone until the new year after she picked up a calf injury in training.
Toone has featured in five games in the Women's Super League this season, but is yet to register a goal or an assist so far this term.
The 25-year-old has missed United's last two games and was not selected in Sarina Wiegman's England squad for their upcoming friendlies against the United States and Switzerland.
She had previously appeared in a staggering 96 straight matches before missing United's WSL showdown with Leicester City last weekend.
But ahead of the Red Devils' meeting with league leaders Chelsea on Sunday, Skinner confirmed they would be without Toone for not only that game, but for the foreseeable future.
"We have three games [after the international break and before Christmas]. There's always the hope, but I think you balance and you go, 'right, if we can get her back, we get her back'," Skinner said.
"But we're just not going to rush her in this moment because it's a calf injury, it's your running mechanism. If we injure the running mechanism, it takes longer.
"So ideally we'd like her back for those games afterwards.
"The projection is she misses this game [Chelsea], she misses the international window, probably misses the first game after that, and then we'll see from there.
"But because it is such a short block of only three games, we'll assess whether it's worth that or actually resetting to come back after Christmas."
Al-Nassr 1-2 Al-Qadsiah: Aubameyang winner ends hosts' unbeaten run
Pierre-Emerick Aubameyang hit the winner as Al-Qadsiah inflicted a 2-1 defeat on Al-Nassr in the Saudi Pro League on Friday to end their 21-match unbeaten run.
Cristiano Ronaldo's seventh league goal of the season put Al-Nassr ahead, but Al-Qadsiah replied swiftly through Julian Quinones before Aubameyang struck the decisive blow.
The hosts drew first blood in the 32nd minute when Ronaldo slotted home after Al-Qadsiah goalkeeper Koen Casteels had spilled a routine catch from Talisca's cross.
However, Al-Nassr's joy was short-lived when Quinones brought down Turki Al Ammar's delivery five minutes later, with the Mexcio winger firing past Bento to level the scores.
Quinones also played a part in the winning goal after brilliantly turning the ball back across goal from Nahitan Nandez's search pass to tee up a simple finish for Aubameyang.
The result sees Al-Qadsiah move level on 22 points with third-placed Al-Nassr, with Al-Shabab also level with them.
Stefano Pioli's Al-Nassr are six points adrift of defending champions and leaders Al-Hilal, who also have a game in hand.
Slot: Guardiola contract is 'good news' for Premier League
Arne Slot believes it is "good news" for the Premier League that Pep Guardiola has signed a contract extension with Manchester City.
The Spaniard signed a two-year extension with the reigning champions and will remain in the dugout at the Etihad until 2027.
He has won 18 major trophies in his time at City, including six Premier League titles, and twice beat Jurgen Klopp's Liverpool to the top-flight crown by a single point.
Guardiola averages 2.35 points per game in the Premier League, the highest rate of any manager (20+ matches). Indeed, he is so far ahead of the next highest (Alex Ferguson's 2.16), he could lose his next 26 league games in a row and would still be top.
Slot is yet to face City since taking over from Klopp at the start of the season, with the rivals' first meeting of the season set for December 1.
"It's good news for City and for the league because everyone wants the best managers and the best players over here," Slot said when asked about Guardiola's new contract.
"He is definitely one of the best managers, maybe the best manager of the league.
"He's won four titles in a row, so it's fair to say he's maybe the best manager in the league.
"On the other hand, they have so many quality players that, if he would have made the choice to leave, I wouldn't have expected them to end up bottom of the league next season."
Other Premier League managers echoed Slot's sentiment, with Brighton boss Fabian Hurzeler, who inflicted a fourth consecutive defeat on City before the international break, saying it is the "best thing that can happen to the Premier League".
"He is one of the best coaches in the world," Hurzeler said. "With his ideas, he always brings something new to the football stage, and it is always a challenge to compete against him.
"He always adapts quickly, always has new ideas and he can always win the game. I'm very happy because we can learn from him and to compete with him is a big, big experience."
Newcastle United boss Eddie Howe added: "He's contributed so much to English football. He's revolutionised how a lot of teams play.
"I think he's incredibly talented, and it's good for the English game that he's still here.
"He will continue to develop and evolve his style, and people will potentially continue to follow that. From the competition side, I think it keeps the Premier League as the best in the world."
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