The Hollywood Reporter said O'Connell is replacing "Heroes" actor Sendhil Ramamurthy, who dropped out of "Rex" because of scheduling issues.
In the pilot, former "Doctor Who" star Tennant plays Rex Alexander, a Chicago lawyer who coaches his clients to represent themselves in court after he starts having panic attacks.
O'Connell, who has starred in TV's "Sliders," "Crossing Jordan" and "Carpoolers," will play Rex's friend and colleague in "Rex," the entertainment industry trade newspaper said.
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