''We're disappointed in our special teams production,'' Fisher said Monday. ''It's the first time I've ever said that.''
The Eagles scored on a blocked punt just 23 seconds into the game.
Later in the first quarter, Armstrong committed an unnecessary roughness penalty while covering a punt that led to a Philadelphia field goal.
Armstrong, signed by the Rams as an undrafted free agent on May 9, 2013, has spent most of his career on special teams. He was expected to push Jo-Lonn Dunbar for playing time at linebacker this season, but he was unable to contend for a spot.
His special teams work had slipped and his penchant for committing unnecessary penalties did not please Fisher.
''I was not happy with (his) production,'' Fisher said. ''It had been addressed with him numerous times. If guys make mistakes over and over, then it's my fault. It just wasn't working out - so we moved on.''
The sloppy special teams play overshadowed a solid late-game rally by the Rams, who fell short and lost 34-28.
Quarterback Austin Davis led a second-half comeback with 21 consecutive points. He was 14 of 18 for 195 yards and two scores in a 16-minute span to key the rally.
Davis' 30-yard TD pass to Kenny Britt with 9:02 remaining capped off the run and trimmed the deficit to six.
St. Louis had a chance to take the lead and drove to the Eagles 47-yard line in the final 42 seconds before three incomplete passes ended the rally.
''We'll carry the momentum from that into practice this week,'' Fisher said. ''We just need to be more consistent and quit turning the football over.
Davis was 29 of 49 for 375 yards and three TDs for a 103.7 rating.
Running back Zac Stacy, who left in the third quarter with a knee injury, is not seriously injured, according to Fisher. Stacy rushed 11 times for 42 yards and committed a fumble in the third quarter that led to the Eagles' last score and a 34-7 lead.
''He may miss some practice time,'' Fisher said. ''But it doesn't look like he'll miss a game.''
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