Symons made his first start for Saints last weekend, but was forced off at half-time in the 17-15 Gallagher Premiership defeat at Bath.
He had a scan on Monday and Boyd says the 'surgeon is going to make a decision in the next couple of days'.
It is a big blow for Symons, who was not able to play during pre-season due to a pectoral injury picked up at former club Gloucester in the final game of last season.
And Boyd said: "It looks like he's got some ACL damage in his knee that the surgeon is going to make a decision on in the next couple of days.
"It doesn't look particularly good for Andy unfortunately. It's pretty grim."
Symons will be added to a list of long-term absentees that already includes centre Rory Hutchinson (knee) and wing Ken Pisi (shoulder).
"Rory is due back around Christmas time," Boyd said.
"When you've got a long season, that's why you need a deep squad.
"Ken Pisi, Andy Symons and Rory are all longish term injuries.
"We've just got to do well with what we've got."
Prop Paul Hill has not been able to get any game time under Boyd due to being forced to have a cyst removed from his shoulder.
But Hill is now on the comeback trail.
"Paul is end of November sort of time, but he's champing at the bit, training hard and looking forward to getting his boots on," Boyd said.
Saints have been light in the second row department due to the absence of new signing Dom Barrow (hamstring) and James Craig (concussion).
But Boyd said: "Dom and James Craig are both probably two or three weeks away.
"We've been a little bit thin in that area, in the lock space, but Courtney (Lawes), Dave Ribbans and Api (Ratuniyarawa) have done a really good job.
"Moony (Alex Moon) had a good game on Monday night for the Wandies, but it will be nice to get those two good, experienced locks back."
Heinrich Brüssow and James Haskell (ankle) missed last Saturday's game at Bath, but they are expected back soon.
"Heinz has had some concussion symptoms after the Harlequins game and he was still going through his protocol before the Bath game," Boyd said.
"James Haskell got a little ankle and back issue that has been grumbling away so it was an opportunity for Jamie Gibson last weekend, and Mitch Eadie was on the bench."
Tom Wood and Reece Marshall came through their comeback unscathed on Monday night, starting in the Wanderers' 29-26 friendly defeat to London Irish at Franklin's Gardens.
And Boyd said: "A couple of the old boys coming back from injury was good, but the thing that really impressed us last night was that there was a continuation of a lot of the young guys sticking their hand up.
"I said to the boys this morning that there's a heck of a lot of those young guys that I've now got faith in.
"If we take some injuries and we lose a bit of form in certain areas, we've got some boys to go to.
"They're doing a really good job in that Wanderers programme."