Centre-half and club captain Langmead, who had been an ever-present in the Town team this season prior to Saturday's clash with Oxford, limped off after 12 minutes with a suspected knee injury.
Right-back Johnson was withdrawn because of a hamstring strain midway through the second half.
"I'm hoping Kelvin will be available for the Torquay game on Friday but it's a little bit too early to tell at this stage," said Northampton manager Aidy Boothroyd following his side's 1-0 home win over the U's.
"He felt something in his knee and he tried to carry on but I thought we should get him off the pitch if there's any doubt at all.
"Thankfully we've got an able deputy in Lee Collins, who was fantastic and contributed to the clean sheet that we kept.
"John Johnson went off because he was feeling his hamstring a little bit as well and we will need to look at that."
Prior to the weekend, striker Alex Nicholls (broken leg) was the Cobblers' only injury absentee, although midfielder Chris Hackett is playing with a slight calf strain.