Waldock, who was on the field for just four minutes before having to be stretched off with a suspected twisted knee against Sale Sharks on Thursday night, had only just returned to action following two months out.
"It's unbelievable luck really," Mallinder is quoted as saying by Clubcall.com. "He's missed most of this season with injury, just worked his way back and he was on the field five minutes.
"It's really, really unfortunate but hopefully it's not too bad. Dom was smiling afterwards. We're hoping it's not anything worse than that a knee twist. Fingers crossed."
Northampton went on to win the Amlin Challenge Cup quarter-final clash 28-14 at the AJ Bell Stadium.