Though there are different variations of the reported injury, Matt Cassel sprained his MCL last year and missed two weeks.
The Chiefs don't comment on injuries but they do release practice reports and the first one is coming this afternoon. 610 Sports Bob Fescoe writes via Twitter that Jackson is the only Chief not on the field during Wednesday's practice.
So you can expect a "Did not practice" next to Jackson's name on the injury report. We'll get the out/doubtful/questionable/probable designation on Friday.