The South Korea international is out with a virus and missed training on Tuesday.
Michael Owen missed training on Tuesday with a groin problem, but is expected to be fit for the Wolfsburg game.
“Ji-sung has a virus,” said Ferguson. “Patrice Evra had it at the weekend. After the Stoke game he felt a bit uneasy – he was in bed on Sunday and didn’t come to training, but he came in for a bit yesterday, came in today [Tuesday] and he should be alright.
“Michael should be alright for tomorrow. He had a tight groin in training yesterday, nothing serious. But obviously for the game tomorrow we didn’t want to be pushing that. Other than that, everyone’s fit.”