Right-back Simpson suffered a hamstring three weeks ago, and goalkeeper Krul has been sidelined for the past three games with an elbow problem.
Alan Pardew is confident the pair will soon be in contention.
Simpson is pencilled in to return to training later this week, while Krul – who suffered his problem on international duty with Holland – is ready to return to the practice field.
Alan Pardew: “I think Danny will train Thursday or Friday, and Tim’s looking for the all-clear to start next week."
United’s visit to Reading on Saturday is likely to come too soon for the pair, whose last appearances for the club came in the September 2 home game against Aston Villa.