Centre back Ben Davies, 25, was signed from second-tier Preston North End on Monday, while a loan deal was done for Schalke's 20-year-old Turkey central defender Ozan Kabak.
Liverpool are without central defenders Virgil van Dijk and Joe Gomez due to long-term injuries and Cameroon international Matip was forced off with an ankle ligament injury in their 3-1 league win at Tottenham Hotspur last week.
"Unfortunately, we found out now after everybody looked at it that Joel will be out for the rest of the season," said manager Juergen Klopp.
"But he will be ready for the start of the next pre-season and finally everything will be sorted."
Klopp has had to play midfielders Fabinho and Jordan Henderson in defence while also relying on young players to fill gaps at the back.