Thomas Christiansen's men have enjoyed a great start to the 2017-18 campaign and they travelled to face Millwall on Saturday sitting at the top of the Championship.
However, they saw their unbeaten run in all competitions ended by the newly promoted Lions, who came out 1-0 victors, although Leeds still lead the way in the second-tier on goal difference.
Aiden O'Brien scored the only goal of the afternoon at The Den on 73 minutes and Leeds' day got even worse in the final 10 minutes when Jansson had to limp off with a hamstring issue.
The defender has been a key figure under Christiansen this term, featuring in seven of their eight league outings, but he could be set for an extended spell on the sidelines.
Leeds will fully assess the 26-year-old on Monday before revealing further details about the full extent of the complaint.