Weatherspoon, who had been held out of organized team activities while recovering from a knee injury, was limited to drills on a side field during practice. He was not visible to the media during Tuesday's session. The team said he was running with the medical staff when the injury occurred.
"We sent Sean to the doctor for some additional testing and evaluation and unfortunately the results showed that he ruptured his Achilles tendon," coach Mike Smith said in a statement released by the team. "Sean had been working extremely hard to get back on the field, but regrettably he will miss the entire 2014 season. He will have a procedure done in the near future and we expect him to make a full recovery."
Weatherspoon was on short-term injured reserve last season after suffering a Lisfranc foot sprain in Week 2. He did not finish the season due to the knee injury.
The former first-round pick from Missouri is heading into the final year of his contract, so his future with the team seems bleak. He played in just seven games last season and has played in all 16 games only once in four seasons.
The Falcons have been working with Paul Worrilow, Joplo Bartu and Akeem Dent as the inside linebackers this offseason. They drafted three players capable of playing inside: Prince Shembo, Marquis Spruill and Yawin Smallwood.
The Falcons lost two players to season-ending Achilles tears last season: defensive end/linebacker Kroy Biermann and defensive tackle Corey Peters. Biermann is back practicing while Peters remains sidelined this offseason.