Williams will serve as the opener ahead of Kumar Rocker for Friday's game against the Rays.
Williams inked a major-league deal with Texas on Tuesday after he was cut by the Nationals last weekend. The right-hander worked in bulk relief with Washington and threw a season-high 63 pitches on two occasions across nine total appearances. Rocker has worked behind an opener twice this season, allowing just two runs on eight hits and three walks with 16 strikeouts across 12.2 innings.