Albion winger Thomas, 27, was taken to Portsmouth by Redknapp in August 2008 and enjoyed their brief time together before he went to Tottenham.
Thomas said: "Harry is a very good manager, a very honest guy and straight forward.
"He's done a superb job and that's probably an understatement.
"He's what you call a players' manager - he knows how to interact and talk to you.
"I was flattered when he signed me for Portsmouth.
"Unfortunately I picked up a serious back injury and then he left so we parted company."