But, having had the opportunity to assess Firmino, Liverpool are now positive about his chances of playing against Everton in the Merseyside derby on Sunday, while he could even feature against Watford.
"He looks good. We were obviously lucky," Klopp told a news conference. "It is not as serious as we thought at first.
"If Bobby goes down and has to leave the pitch, it's a bad sign because he's a pretty hard boy.
"For tomorrow, it will be very close, but [he has] a big chance for the derby. Because it’s Bobby, I would say yes [he has a chance for Watford] but I don’t know if I would use that chance because we play on Wednesday after playing on Sunday.
"But writing Bobby off is a big mistake, because he recovers pretty quick."
Firmino has been an ever-present for Klopp this season, with the Brazilian having made 27 league appearances thus in 2018-19, scoring nine goals.
Asked how he would cope if Firmino isn't fit, Klopp added: "Somebody else will play!
"That’s the nature of playing for a club like Liverpool. You have a few players that don’t have 500,000 minutes before they start the next game, but they still have a lot of quality.
"If we have to change a few things, we will change. We changed a couple of things during the season, but obviously Bobby was pretty much always involved.
"That’s not the case this time, but we have the options which we will choose for the start, one of them, and maybe during the game a second one.
"If there’s anything good in [the situation] it’s that Watford have no clue what we will do, so we will not talk about that."
The Reds were able to move a point ahead of Manchester City over the weekend, with their goalless draw at Old Trafford moving them onto 66 points.
City, meanwhile, face West Ham at the Etihad Stadium on Wednesday fresh from their Carabao Cup final win.