"Wow," Washington said. "The ball was coming out of his hand really good. The arm strength is picking up."
Washington said Webb threw 60 to 65 pitches off flat ground during a 17-minute session Thursday.
The Rangers decided they wanted the right-hander to build up arm strength before getting back on a mound after his short bullpen session during the first workout a week ago.
The former NL Cy Young Award winner hasn't pitched in a major league game since the 2009 opener for Arizona because of shoulder surgery.
Washington said Webb could throw off a mound Sunday or Monday "if everything keeps progressing."
Webb is set to throw again Friday.
Information from The Associated Press was used in this report.