Jackson has the flu, Jeff Zrebiec of The Athletic reports.
It doesn't seem to be a serious case, considering Jackson was on the sideline for most of Sunday's 28-10 win over the Steelers. He and a few other key players were held out of the regular-season finale, with Baltimore preparing for a divisional-round playoff matchup against the Texans, Bills or Titans on Jan. 11. Coach John Harbaugh expects Jackson to participate in practice Tuesday, per Jamison Hensley of ESPN.com.