The 33-year-old Hill returns to the Cubs after spending all or part of the past five seasons in the organization from 2007-11. He batted .208 with seven home runs and 63 RBI in 252 games with the Cubs during his first stint.
Hill was batting .195 this season with a homer and five RBIs for Pensacola -- the Reds' Double-A affiliate.
Cubs starting catcher Geovany Soto had left knee surgery Friday to repair a torn meniscus and was placed on the 15-day disabled list Saturday. Backup Wellington Castillo has a sprained knee ligament and will be out four to seven days. On Friday, the Cubs called up catcher Blake Lallio.
Wells was 0-1 with a 6.23 ERA in two starts with Chicago earlier this season.