Hamels felt discomfort following a workout with a medicine ball. Hamels has not reported to the team and is in Dallas.
The 36-year-old Hamels signed a one-year, $18 million deal with the Braves in December.
Before he suffered an oblique strain last season that sent him to the injured list, his ERA was hovering around 3.00 for the Chicago Cubs. In 10 starts for the Cubs after returning, Hamels compiled a 5.79 ERA. He was never the same, mostly because of his mechanics. He finished the year with a 7-7 record, 3.81 ERA and 143 strikeouts.
The Braves also announced Wednesday that they have have agreed to a minor league contract with reliever Josh Tomlin that includes an invitation to the big league camp.
Tomlin, a 35-year-old right-hander, returns to the Braves after making a career-high 51 appearances in 2019. He was 2-1 with a 3.74 ERA working mostly in long and middle relief, though he did have the first two saves of his 10-year career.
Before coming to Atlanta, Tomlin pitched nine seasons -- mostly as a starter -- with Cleveland. He had three seasons with at least 10 wins, including a 13-9 mark in 2016.
Tomlin provides additional depth to a Braves bullpen that was bolstered last season by trade-deadline deals for Mark Melancon, Shane Greene and Chris Martin and the November free-agent signing of San Francisco Giants closer Will Smith to a three-year, $40 million contract.
The Associated Press contributed to this report.