Lee (oblique) has resumed throwing off flat ground and is expected to pitch off a mound in the coming days, Roch Kubatko of MASN Sports reports.
The left-hander has been idle for a little over a month with a right oblique strain, which ended his bid for making the Opening Day roster. The Orioles optioned Lee to Triple-A Norfolk in March, so he's expected to hit the club's 7-day disabled list when its season begins April 6.