The veteran righthander was scheduled to pitch against the Houston Astros on Friday.
Armas last pitched on Sunday against the Cincinnati Reds, and it was his worst outing of the season, giving up five earned runs in six innings. The Reds also hit four home runs against the Armas, who saw his ERA climb from 1.44 to 2.61.
The Expos then recalled righthander Claudio Vargas from Triple-A Edmonton of the International League. He will replace Armas on Saturday.
According to Manager Frank Robinson, Armas' arm felt fine after his last outing, but when he woke up the next day, he could barely lift it.
"He was throwing the ball very well with the exception of his last outing," Robinson said. "I was looking for big things out of him this year. He had gotten off to such a great start. We are going to miss him. We are doing the best thing for him. The doctors recommend that he go on the disabled list where he'll take it slow and easy. Within 15 days or so, he should be fine."
According to Robinson, Armas, who declined to talk to media, "battled" not to go on the DL. He felt that he could have missed just one turn.
"Mentally, he's down," Robinson said. "He did not want it to happen. He battled (in the manager's office) the whole time. I can understand that. Once he looks back and takes a look at it, he'll know it's the best thing for him and the ballclub. I like the competitiveness in him -- that he still wants to try to go out there."
Robinson said that the Expos felt that Vargas was "throwing the ball the best" in the minor leagues. The stats back that up.
Vargas, 23, was a combined 1-0 with a 1.66 ERA for Edmonton and Double-A Harrisburg in 2003. Vargas' last start came Sunday for Harrisburg, where he scattered three hits in six scoreless innings against Double-A Akron.