A knee injury will rule out Barcelona goalkeeper Marc-André ter Stegen for months after he was carried off on a stretcher during Sunday's 5-1 LaLiga win at Villarreal, a source confirmed to ESPN.

Barça coach Hansi Flick confirmed the injury appears to be "huge and serious," but added he could not offer any timeframe until further tests have been carried out.

Radio station RAC1 reported Ter Stegen, who was taken to a hospital close to Villarreal's Estadio de la Cerámica, could even be out for as long as eight months, which would rule him out for the rest of the season.

"It doesn't look good for Marc," Flick told Spanish television after the game. "We have to wait to know the full diagnosis of the injury, but it's true it does appear to be really serious."

Ter Stegen, 32, collapsed to the ground after claiming a corner at the end of the first half and it was immediately clear that he would not be able to continue. Iñaki Peña replaced him.

The injury to the German international tainted what was otherwise a good day for Barça, who scored five times to make it six wins from six in LaLiga this season and remain four points clear at the top of the table.

"This injury makes us sad," Flick later added in a news conference. "When you win 5-1 at Villarreal, the fans here in the stadium, also those watching on television, saw a fantastic match.

"Also from Villarreal, [they played with] unbelievable speed and were dynamic in the match. With great goals from us, we are very happy with the 5-1, but at the end we are sad because I think the injury will be huge, but we have to wait for information."

Ter Stegen joins a lengthy injury list at Barça, with Ronald Araújo, Andreas Christensen, Marc Bernal, Frenkie de Jong, Gavi, Fermín López and Dani Olmo all currently out of action as well.

Despite that, Barça have managed to win all six of their opening LaLiga games, with Flick once again putting his faith in youngsters at Villarreal by handing first starts of the season to Sergi Domínguez and Pablo Torre, while Gerard Martín also came into the side at left-back.

"At the end, the team has good character," Flick added of Barça's ability to overcome setbacks so far this season.

"I would say the attitude is unbelievably good. I love it, every day, the mixture between very focused and also enjoying playing football. This is what we like.

"We have to accept it [the injuries]. Marc today it was an accident. It's really tough, but we have enough players. I am really proud of the team and how they played. Everyone who came in did fantastic. It was a big win for us."

A brace from Robert Lewandowski -- his fifth and sixth goals in LaLiga this season -- gave Barça a 2-0 lead at Villarreal before Ayoze Pérez pulled a goal back for the break.

The home side then flew out of the traps in the second half, having the second of three goals disallowed for offside and hitting the crossbar before Pablo Torre restored Barça's two-goal lead having set up the opener for Lewandowski.

Raphinha then scored twice to wrap up the three points for Barça, the second after a sublime Lamine Yamal pass, after Thierno Barry's header had been ruled out for offside.

There were some unsavoury moments in the latter stages of the match, with Yamal subjected to some rough challenges as the Villarreal players grew frustrated.

"We have to protect every player," Flick said when asked about how Yamal was treated. "I haven't seen the situations so clear, maybe when analyse the game I can say more, but I don't want to speak too much about Lamine.

"He's doing great, he has a lot of quality, but also the other players are. Lewandowski and Raphinha with goals today, our centre defenders, our midfield is really creative. We are a team in defence and attack, a compact unit... a team that loves to play for each other."