Vitezslav Jaros, 24, suffered a severe ACL injury in training and will undergo surgery, ruling him out for the rest of the season.
The Czech stopper was Ajax's starting goalkeeper for the majority of the season after arriving on loan from Liverpool in the summer, making 26 appearances across all competitions and conceding 42 goals along the way.
Indonesian international Maarten Paes is expected to take over as Ajax's starting goalkeeper. The 27-year-old arrived from MLS outfit FC Dallas in January.
Injury issues
Jaros' injury is the second severe injury Ajax have suffered in recent days, with former Arsenal left-back Oleksandr Zinchenko also ruled out for the rest of the season with a knee injury suffered in last weekend's 4-1 win over Fortuna Sittard.