Williams was designated for assignment by Cincinnati in May after going 2-3 with a 7.20 ERA in eight starts, and was acquired by the Mets for minor league pitcher Robert Manuel.
He is 2-2 with a 3.68 ERA at Triple-A Norfolk, and was chosen over left-hander Oliver Perez for the start.
"Right now, he's probably throwing the best of everyone down there, and he's on turn," manager Willie Randolph said.
REPLAY: David Wright, on a 3-for-24 slide going into the game, drove in two runs with a double and single in the Mets' 6-3 victory Friday over Colorado.
Steve Trachsel went seven innings in winning his 12th game of the season.
The Mets will honor their 1986 World Series championship team Saturday night at Shea Stadium.
C Ramon Castro, on the disabled list with a strained left rib cage, has no timetable for his return.
2B Jose Valentin is day-to-day with a slightly strained right hamstring. Valentin began on the season as a role player, but his consistent play when Kaz Matsui went on the disabled list prompted the Mets to trade Matsui to Colorado. The Mets will go with Chris Woodward at second base.
BY THE NUMBERS: 7 -- Mets who have hit three homers in a game, with SS Jose Reyes being the latest, Aug. 15 at Philadelphia.