Ryan Shawcross is in self-isolation after a positive Covid-19 test at Stoke City.
The club captain would have been expected to challenge with Danny Batth to fill a hole left by James Chester (suspension) for tomorrow’s trip to Rotherham United.
But a positive result at the end of last week ruled him out of a place in the squad for Saturday’s draw at Blackburn and he remains at home.
Clubs are now carrying out twice-weekly tests, funded by the PFA, to try to mitigate the spread of the disease.
Michael O’Neill said: “Ryan unfortunately has had a positive Covid test and his isolation has started.
“We are now in a scenario when we are tested twice a week, Ryan was tested on Thursday and unfortunately he had a positive test.”
Shawcross has worked his way back into the first team fold in recent weeks after the best part of 18 months in the treatment room, starting when he broke a leg in pre-season 2019.
He has made two sub appearances and completed 90 minutes in the FA Cup against Leicester.
Chester serves a one-match ban after being shown a straight red card for a foul on Adam Armstrong at Blackburn.
O’Neill said: “In terms of centre-backs, Danny Batth and Liam Lindsay are both available and fit.
“In terms of other injuries, Morgan Fox is still out, Lee Gregory is still out and Tyrese (Campbell) is longer term.”
Latest EFL Covid-19 guidelines for players
Positive test result with symptoms: Must exclude for seven days minimum off-site or until symptoms resolve. If necessary for symptom resolution, individuals to remain away for a further seven days whilst undertaking a gradual return to activity.
Positive test result with no symptoms: Must first confirm if they have experienced any symptoms in previous month. If they have not experienced any symptoms, they must exclude from training for 10 days and return on day 11.
Through days 11-14 they will be under strict surveillance and if the individual does develop symptoms they must then exclude for 7 days from this point.
Negative test result with symptoms: Will return to training at resolution of symptoms
Negative test result with no symptoms: Allowed to continue training as usual
If an individual tests positive, they do not get tested again for 28 days as a further positive test is linked to the previous infection in that period. Currently there is no scientific evidence for reinfection.
During stage one: Only the individual player or member of staff is required to self-isolate following a positive test result. This is due to social distancing measures in place.
During stage two: In addition to the individual player or member of staff required to self-isolate, any close contacts of that person must self-isolate for 14 days. There is no requirement to exclude other players or members of staff, unless deemed by club medical staff to have been in prolonged close contact with the individual who has tested positive
Prolonged close contact is when players or staff members have been within the less than two metres apart for 15 minutes or more and advice will be taken from Public Health England in regards to contact tracing.