LHP Tommy Hottovy was designated for assignment to open a spot on the 40-man roster.
Jenks has pain his left mid-back area, the same injury that put him on the disabled list on June 8. He traveled back to Boston yesterday and will be examined today.
Jenks has appeared in just 19 games and has a 6.32 earned run average since being signed to a two-year, $12 million contract in December.
The Red Sox envisioned Jenks as a reliever who would lock down the seventh or eighth inning. But he has rarely pitched and pitched poorly when he has.
“That’s an important part of our bullpen, that complement to [Daniel] Bard,” Terry Francona said. “That other arm is a huge arm. He’s not just a thrower. He can manipulate the ball; he can spin it. He’s got a good feel for the game. We just haven’t had him out there healthy and we miss that.”
Williams, 35, has been with seven organizations since being drafted by the Cubs in 1997. The Red Sox signed him to a minor league contract over the winter.
Williams had a 1.29 ERA in 19 appearances for Pawtucket, striking out 26 in 21 innings. Lefthanded hitters were hitting .136 against him.
“He’s been dominating lefthand hitters. Velocity has been tremendous,” Francona said. “It gives us that second lefty where we can match up.”