The Guinea international also looks certain to miss all of the 2019 Africa Cup of Nations, which is set to kick off in June.
Klopp told reporters, per the Liverpool Echo's James Pearce:
Keita started in Wednesday's 3-0 defeat to Barcelona in the first leg of their UEFA Champions League semi-final but had to come off after only 24 minutes.
The Reds head to Newcastle United on Saturday before Barca travel to Anfield for the second half of their Champions League clash on Tuesday.
Club captain Jordan Henderson was the only midfield cover on Klopp's bench, coming on to partner James Milner and Fabinho in a central three that lacked creativity at times. Georginio Wijnaldum was pushed into a more advanced role after Roberto Firmino missed out on a start following injury.
The injury is poorly timed for Keita, who has endured a slow transition into Liverpool's team following a transfer last summer but was just beginning to find his feet.
After nine months at Anfield without a goal, the 24-year-old opened his Liverpool account in April, scoring three times in his five appearances leading up to Wednesday's trip to the Camp Nou.
Firmino came off the bench to provide a much-needed focal point up front with only 10 minutes remaining.
The Brazilian is one of Klopp's vital tools in attack, but he may not even be ready for the trip to Newcastle, per the Mirror's David Maddock:
Reigning Premier League champions Manchester City boast a one-point lead over the Reds entering the penultimate week of the campaign. The Citizens are now just four better off in terms of goal difference, though, after formerly leading by a much larger margin in that regard.
The Liverpool manager sounded more positive about Alex Oxlade-Chamberlain's chances of landing a place in his XI at St. James' Park:
This season has produced one of the closest Premier League title battles in living memory, so close in fact that the proposition of a play-off has been discussed to win the title. For this to occur, teams must finish level on points, goal difference and goals scored.
City have scored 90 league goals this season to Liverpool's 84 with two games remaining, but Klopp relished the thought of a final showdown to decide who takes the crown: "We need to score five more goals than City. Wow. When would it be? Between the last game and FA Cup or after that? It would fit with the season, a big showdown at high noon somewhere."
Keita will have no further part to play in their title push, while Guinea's chances of success at the Africa Cup of Nations next month have also been rocked by the loss of their best player.