The Brazilian had to be stretchered off after going down on 34 minutes having been caught by the follow-through of Didier Ndong's clearance.
The 24-year-old was in serious discomfort on his way of the pitch and later left Anfield on crutches, his right foot encased in a protective boot.
Scans from the Spire Hospital on Monday have since revealed damage to Coutinho's ankle ligaments, which will keep him out of the whole of the Reds' hectic Christmas schedule.
The results, which realise Liverpool's initial fears, come as a huge setback. The inspirational Coutinho has been the star turn of an attack hailed as one of the best in the Premier League so far this term.
With six goals and six assists in 13 matches across all competitions, the No.10 had been enjoying the best run of form of his Liverpool career - but suddenly faces a frustrating month-and-a-half out of action.
However, Liverpool will be thankful that - if reports are to be believed - Coutinho does not require surgery, which would dramatically lengthen his time out injured.
Coutinho to miss busiest period of Reds' season
Coutinho is expected to miss Premier League fixtures against Bournemouth, West Ham United, Middlesbrough, Stoke City and Sunderland as well as the Merseyside Derby against Everton at Goodison Park and a heavyweight New Years' Eve clash against Manchester City.
He will also be absent for the third round of the FA Cup and potentially the first leg of the EFL Cup semi-finals, if Liverpool manage to see off the challenge of Championship side Leeds United on Tuesday night in order to reach the last four.
His first game back, depending on how well his recovery goes, could be the short trip across the M62 to face arch rivals Manchester United in mid-January.
Jürgen Klopp now faces having to rejig a deadly front-three - though has the likes of Divock Origi and Daniel Sturridge to call upon from their reserves.
The German could also be forced to use the transfer market to strengthen his attacking options, with Danny Ings out for the rest of the season with another cruel knee injury and Sadio Mane absent for the whole month of January due to the African Cup of Nations.
Liverpool have been linked with moves for Spartak Moscow winger Quincy Promes, Borussia Dortmund's American teenage starlet Christian Pulisic and FC Rostov forward Sardar Azmoun.
Earlier on Monday, addressing the press in his pre-Leeds briefing, Klopp told reporters: "What I can say is it will be too long, because the best would be that he's not out at all. But that's not really likely.
"We have to wait for the final scan and we know how long it will be probably. We have to take it like it is."