The Midlands club, whose other overseas player is Australia batsman Phil Hughes, had hoped to have Ajmal, England's tormentor-in-chief during Pakistan's recent 3-0 Test series win in the United Arab Emirates, back at New Road.
However, the confirmation of Pakistan's tour of Sri Lanka has left the club looking for another overseas player.
"Following the recent announcement that Pakistan's tour of Sri Lanka will proceed, Saeed Ajmal will be unavailable for Worcestershire's T20 campaign," the county said in a statement Thursday.
"Ajmal had signed a contract to appear in Worcestershire CCC's T20 run at a time when the Pakistan tour to Sri Lanka was not agreed in the ICC schedule.
"But it is now understood that it is likely to take place between late May and mid-July 2012, ruling Ajmal out of the county's T20 campaign."
Worcestershire director of cricket, Steve Rhodes, said: "Losing a player of Saeed's quality is a big disappointment.
"However, we are extremely active in finding a replacement for the Twenty20," the former England wicketkeeper added.
Ajmal took 24 wickets during Pakistan's Test rout of England.
