Isaksson has sustained a hairline fracture of the thumb which forced him to miss Saturday's friendly with Valencia.
"Unfortunately we had two goalkeepers get injured within two days," Eriksson told club website mcfc.co.uk.
"Joe Hart should be back training on Tuesday or Wednesday, a maximum of a week.
"Isaksson is worse, though, the medical staff say he will be away for four weeks.
"It's not the best way to start the Premier League, starting with two very young goalkeepers.
"We will look at it on Monday. Unfortunately it's more or less a must that we bring another keeper in."