Reader (illness) was listed as a non-participant on Monday's practice estimation, Tim Twentyman of the Lions' official site reports.
Reader appears to be dealing with an illness following the Lions' Week 12 win over the Colts, and he'll likely need to practice in a limited capacity either Tuesday or Wednesday in order to play in Thursday's divisional matchup against the Bears. If the first-year Lion is unable to do so, Levi Onwuzurike and Mekhi Wingo could see additional work with Detroit's first-team defensive line.