Cummings was withdrawn at half time at Highbury after injuring his groin. He joins Niall Mason on the sidelines who is recovering from a toe injury.
Auxiliary right back Matty Blair missed the game due to illness and it is not known whether he will be deemed fit enough for Saturday's trip to Gillingham.
Herbie Kane was forced to fill in at right back after Cummings' withdrawal but looked to struggle in the position.
'Shaun felt his groin so we had to jingle it around a bit and put our best attacking midfielder at right back which was frustrating,' McCann said.
'It is what it is. We've got two full backs now injured with Niall Mason and Shaun Cummings and Matty Blair is ill.
'Niall will definitely not be fit.
'Matty should be back. He wasn't too bad but we just didn't want anything spreading so we sent him home from the hotel on Wednesday morning.
'We're not sure how Shaun is so we'll have to see.'
Joe Wright, recovering from an ankle injury, will not be fit enough to return at Gillingham.