The move was timed for the return of all-star right-hander Chris Young, who spent time on the 15-day disabled list after suffering a strained left oblique muscle in his start against Colorado July 24.
The move could signal an end to the 20-year major league career of Wells, 44, who was 5-8 this season with 33 walks, 63 strikeouts and an ERA of 5.54 in 118 2/3 innings.
Wells was hammered for 26 earned runs in his last 26 2/3 innings.
"He absolutely still wants to pitch," said San Diego General Manager Kevin Towers, who spoke with Wells after the pitcher yielded seven runs and 11 hits against the Cardinals Monday night.
Towers said several teams have expressed interest in Wells for the stretch run.