Symons, who joined from Gloucester, suffered the knee injury in the first half of last month's Gallagher Premiership defeat at Bath.
And he has been forced to have surgery, dealing Saints a second big blow in the space of as many weeks, following the news that Harry Mallinder is also set to be out for the rest of the season after having a knee operation.
Tongan centre Nafi Tuitavake is another who will have to go under the knife to repair a shoulder problem, while wing Ken Pisi will not be back until February with his own shoulder issue.
It leaves Saints' back-line options stretched to the limit, with Boyd admitting he may have to bring in some injury cover if things get any worse.
"We're close to creaking but not there yet," Boyd said at Saints' season ticket holders forum on Monday night.
"Harry's season is probably finished, Ken Pisi has done his shoulder and he's not back until February.
"Andy Symons' season has finished because he's had an ACL reconstruction.
"Nafi Tuitavake has got to go under the knife and have some attention to his shoulder.
"There's six all out in the one space so we're getting pretty thin in that area, but the guys who are left are going well.
"We're very blessed to be left with a couple of young boys: Fraser Dingwall, who played well last weekend, and Fraser Strachan, who came on.
"We're putting some faith in our youth to fill that hole for now, but we're not far away from having to call in the cavalry."