Boss Chris Boyd says Gibson looks likely to be out for around three months in a big injury blow for the club.
With Heinrich Brüssow having been forced to retire earlier this season due to his injury problems, Saints are stretched in the back row.
Courtney Lawes played at six last weekend as Tom Wood was out with illness, lining up alongside staple features Lewis Ludlam and Teimana Harrison.
JJ Tonks, Alex Coles and Mitch Eadie are among the other options in the back row.
When asked about Gibson, Boyd said: "He's got a broken ankle so he's out for three months and you can put him on the long-term list for now.
"We played Courtney at six last week and he continues to be an option for us.
"We've also got JJ Tonks, Mitch Eadie and Alex Coles so we've got some good choices there.
"What will be will be."
Gibson joins knee injury victim Alex Mitchell on the long-term absentee list, but things are improving for Saints overall.
"Apart from Gibbo, the only one left who started the season injured to be any length of time away is Alex Mitchell, who is probably another four or five weeks," Boyd said.
Matt Proctor was forced off due to concussion during the defeat at Sale Sharks last Saturday.
But Boyd is hopeful that the New Zealand centre will be able to face Gloucester this weekend.
"He's got to go through a process," Boyd said.
"It makes it a bit tricky with Christmas Day because he might have to end up coming in to do a bike session on Christmas Day.
"But he should be not too far away, as is Reece Marshall (hamstring), as is James Fish (head)."
There is also good news in the form of scrum-half Henry Taylor, who is on his way back from a broken thumb, and David Ribbans, who has been out with a back problem.
"Henry is pretty good," Boyd said.
"We'll find out tomorrow where he sits from his injury point of view but he's possibly available for the weekend.
"David Ribbans is this week or next week - he's not far away."