Barcelona faces two long months without Lionel Messi after its star forward tore a ligament in his left knee during the defending champion's Spanish league match against Las Palmas on Saturday.
The Catalan club confirmed the injury after Messi had tests at a nearby Barcelona hospital. Barcelona said it expects Messi to be out "seven to eight weeks."
Messi went down after colliding with Las Palmas defender Pedro Bigas in the third minute.
He writhed on the ground in front of the Las Palmas goal before play was stopped. Following medical attention, the Argentina star went back on, but two minutes later asked to be replaced in the eighth after carefully trotting around and testing his leg with a pair of passes.
Messi walked off by himself as Munir El Haddadi replaced the four-time world player of the year in front of a subdued Camp Nou crowd of almost 75,000.
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Spanish television showed images of Messi leaving the hospital with a limp as he got into a car.
His coach and teammates showed their concern after the final whistle at Barcelona's 2-1 victory over newly promoted Las Palmas.
"I'm sad anytime a player gets injured, and if it is Messi even more so considering the weight he has on the team," Barcelona coach Luis Enrique said. "Whenever there is an injury, the important thing is the team. We are a team and will pull through this together."
Among several other matches, Messi will likely miss the first "clasico" of the season against Real Madrid on Nov. 21.
He will definitely watch as Barcelona hosts Bayer Leverkusen in the Champions League group stage on Tuesday. Messi became the first player to score five goals in a single Champions League match in 2012 to fuel Barcelona's 7-1 romp over Leverkusen.
Known as a durable player unafraid of tough tacklers, Messi was limited by a persistent series of hamstring injuries during 2013, but returned to his best form last season and scored 58 goals.
The 28-year-old Messi led Barcelona to a sweep of the Champions League, Spanish league and Copa del Rey titles last campaign.
He is the all-time highest scorer for Barcelona, and also has scored more goals in the Spanish league than anyone else.
"He is the best player in the world, so of course we will notice his absence," Barcelona midfielder Sergio Busquets said.
Barcelona had already lost attacking midfielder Rafina to a ligament tear for an expected six months. Also, Pedro Rodriguez left during the offseason for Chelsea, meaning Luis Suarez and Neymar will have to carry the scoring load for Barcelona until Messi returns. The club can field Arda Turan and fellow new arrival Aleix Vidal after the ban on its new players expires in January.
Luis Enrique rejected the idea that his team would have to "survive" without Messi.
"This is a test and an extra motivation for this team," the second-year coach said. "We have to accept the challenge and improve our performances because we are capable of it.
"We won't play differently, we will play like always. Messi freed up our team in many ways ... (but) we have experienced players who can decide matches."