Liverpool suffered an injury blow in the 0-0 draw at Manchester United with Roberto Firmino leaving Old Trafford on crutches.
The Brazilian looked to twist his right ankle when passing the ball in the 28th minute of the Premier League stalemate with their fierce rivals.
He was replaced three minutes later by Daniel Sturridge, who tried a speculative shot from outside the penalty area that did not trouble David de Gea, shortly after coming on.
Manchester United lost three players of their own - Ander Herrera, Jesse Lingard and Juan Mata - with injuries before half-time.
Asked after the game what he knew about Firmino's injury, Liverpool boss Jurgen Klopp said: "Unfortunately I have no clue. It was something with the ankle, that's for sure.
"Bobby said first he can carry on, then he showed it doesn't work but I don't know exactly.
"It's Bobby, so he will be treated along with all the others but first we have to know what it is exactly and we don't know at the moment.
"It was really strange with all these injuries.
"Everyone was like 'be careful', then it happened to us with Bobby, which was a catastrophe for us as well.
"We only had one [injury] at least, hopefully it's not serious."
Speaking during the game, Sky Sports, pundit Jamie Carragher said: "The one real clever player with his passing in that front six is probably Firmino and he's the one who's going to come off.
"I think a lot of Liverpool supporters will tell you they still lack someone who is capable of unlocking a door with a clever pass."
The game finished goalless to leave Liverpool a point clear at the top of the table.