The 24-year-old, who scored 16 goals for the Amsterdam club last season, was diagnosed with a collapsed right lung last week after complaining of persistent tightness in his chest and coughing.
De Jong remained in hospital for a few days last week as his lung was drained before he was released and he watched the Eredivisie champions defeat Feyenoord 2-1 at the Amsterdam ArenA on Sunday.
But the hospital has since confirmed a new drain was necessary, and the striker posted on his official Twitter account: "It is certainly a drain... So again a few days in the hospital..."
The club also confirmed the captain's readmission to hospital through their official website while stressing the procedure would have "no significant impact on the road to recovery".
That period is likely to last about six weeks which means De Jong will be ruled out of Ajax's opening Champions League group-stage match next month.