Grabara is set to miss Poland's Nations League group stage matches against Bosnia and Herzegovina, with media reports saying the keeper faces up to seven months on the sidelines.
"I’m not going to lie, this is not something I ever wanted to post, but life comes at you fast sometimes," Grabara wrote on Instagram, posting a picture of himself on a hospital bed.
After joining Juventus on loan from Wolfsburg in August, Grabara debuted for the Serie A side on Thursday, providing an assist and winning the player-of-the-match award in a 5-0 Europa League win over NEC Nijmegen.
On Saturday, however, he felt a strain in his right knee during training, and tests revealed he had suffered an ACL tear.
Local media reported that Juventus were nearing a deal with 37-year-old Brazilian Neto, who played for the club from 2015 to 2017, as Grabara's replacement.