Roberto Firmino is a major doubt for Wednesday's Premier League meeting with Watford after coming off injured at Old Trafford.
The Brazil international left Manchester on crutches after an ankle injury forced him from the field during the 0-0 draw with Manchester United on Sunday.
Jurgen Klopp said: "I don't know exactly. It is Bobby, so it will be quicker than with all the others but first we have to know what it is exactly."
Daniel Sturridge, who came on at Old Trafford, could deputise if Firmino fails to recover in time for Watford's visit, though Klopp could also opt to reconfigure his front line.
Meanwhile, Andrew Robertson is a "bit sore" following a knock to his arm against United, and Dejan Lovren remains a major doubt with a hamstring injury.
Joe Gomez and Alex Oxlade-Chamberlain are continuing their respective recoveries, but both remain some way from a return to first-team action.
Liverpool XI [4-3-3]: Alisson, Alexander-Arnold, Matip, Van Dijk, Robertson, Keita, Fabinho, Wijnaldum, Mane, Sturridge, Salah
