“It’s not too cool,” said Klopp.
“It’s a calf injury which takes Alisson out for a while.”
“I don’t want to make now an exact prognosis on when he will be back – it takes a couple weeks for sure, and we have to see,” said Klopp.
“I saw already like ‘six weeks’ and stuff like this, but Ali was not a lot injured in his career so I would like to wait a little bit to see how he develops in that process now, but he is not in for the next few weeks.”
Lonergan is available on a free transfer having left Middlesbrough in the summer.
The 35-year-old initially joined the Reds during pre-season as cover when Simon Mignolet, who has since left, was their only available keeper, with Alisson on holiday following international duty and Kelleher injured.
Another back-up keeper, Kamil Grabara, is currently on a season-long loan at Championship side Huddersfield.