Jim Mallinder confirmed Luther Burrell sustained a dead leg in Saturday's win against Connacht.
Burrell was forced off after just 22 minutes following a huge collision while charging at the opposition defence.
Rory Hutchinson replaced the 29-year-old and Saints went on to claim a 21-15 victory at Franklin's Gardens.
Jim Mallinder's men will now face Stade Français in a shootout for a Champions Cup place at the Gardens on Friday night.
"Luther got a bit of a dead leg and George (North) a bit of a bloody nose, but apart form that we've come out pretty much unscathed," Mallinder said.
"For this time of the season, that's pretty good.
"We're pleased to have won against Connacht and we'll really look forward to finishing the season well - that's what it's all about."