The news is hardly ideal for the Lions after last Saturday's 50-17 humbling at the hands of the Hurricanes at Ellis Park.
Jantjies limped off in the second half and the frontrunner for the Springbok fly-half slot against Ireland next month now faces a nervous wait to discover the extent of the damage.
“Elton has a little bit of a knee, at this stage it is a medial ligament but we are not sure of the extent of it," Ackermann told Supersport.
"The moment we are sure we will send out the report but we will probably only know by Tuesday because of the holidays. The doctor said he will only be able to do a scan on Tuesday."
Ackermann also provided a fitness update on the condition of prop Julian Redelinghuys (wrist) and hooker Malcolm Marx (neck) who he hopes will both be available when the Lions return from their bye week to face the Blues.