He started Tuesday's 11-6 win over the Atlanta Braves but threw only 30 pitches before exiting in the second inning.
The IL designation comes after Strasburg underwent an MRI.
Following the Nationals' 2019 World Series triumph, the franchise handed the right-hander a seven-year, $245 million contract. He had finished fifth in the National League Cy Young voting and won the World Series MVP. It seemed like a reasonable investment at the time.
Unfortunately for the Nationals, the deal is looking more regrettable now.
Strasburg was limited to two games in 2020, eventually undergoing surgery for carpal tunnel neuritis in his throwing hand. His 2021 season didn't start off much better as he left his second appearance after allowing seven runs over four innings.
Washington placed the 32-year-old on the injured list with shoulder inflammation in April.
Losing Strasburg to an injury again obviously hurts the Nationals' playoff chances, but their concerns likely extend beyond this season. The three-time All-Star is signed through 2026, so his shoulder problems in particular raise obvious questions about his long-term value.