Thompson struggled badly with his control in the early stages of the season, and hasn’t pitched in the big leagues since May 17. He posted a 2-2 record with a 4.22 ERA in 15 appearances, with 16 strikeouts and 14 walks in 21.1 innings.
This season, he has posted a 9.36 ERA in Iowa, but after a stint on the injured list this summer he has allowed two earned runs in his last 8.1 innings, according to the Cubs.
In a corresponding roster move, the Cubs optioned pitcher Michael Rucker to Triple-A.
In 35 appearances, Rucker has a 4.91 ERA and a 2-1 record, with 40 strikeouts and 19 walks in 40.1 innings.
The Cubs will start a four-game series against the Pirates on Thursday night, with Justin Steele toeing the rubber against Pittsburgh. Former Cubs pitcher Rob Zastryzny will pitch for the Pirates.