It was the same knee that Gundogan had dislocated while at Borussia Dortmund in March that cost the German worldwide a place in the European Championship.
But he started the season crocked and it looks like he's facing another long spell on the sidelines.
"We're going to miss him I think for a long time".
Manchester City midfielder Ilkay Gundogan is set to miss several months of action after suffering a bad knee injury during Wednesday's 2-0 win over Watford.
The 26-year-old, however, had impressed with City in his early days, scoring five goals in the process.
"Arsenal are going for the title, but there is a lot of the competition still to play, and you can win or lose against any team".
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"I am so, so sad for Gundogan", said Guardiola.
"Obviously we are all behind him and follow what he says". Reds boss Klopp admitted it was hard telling the 23-year-old that he had been benched in favour of Belgium worldwide Simon Mignolet for the comprehensive victory at the Riverside Stadium that pushed the Anfield side up to second in the Premier League. "We need his spirit".
"On the pitch and in the dressing room he is important for us".
After the break, and with a different approach from the visitors, they looked more threatening and could have levelled through Odion Ighalo or Etienne Capoue, but were made to pay when David Silva added a second late on. "It's not the best moment for him at present". I'm really happy with our goalkeeper situation, but there's no reason to push one through, we want to develop the players.
Klopp denied he had bowed to pundit pressure in the wake of criticism aimed at Karius from former Manchester United defenders Phil and Gary Neville, who had both criticised the keeper in recent weeks. He will have a scan on Thursday to assess the full extent of the injury. "And we need to understand what it is exactly, because we still don't know".
"If that was an Englishman or a foreign manager making his way in the game, people would be asking 'who is this guy and what's he playing at?' But because it is Pep and his record tells you he usually gets it right, nobody is kicking off too much".