Serbian centre-back Vidic, who returned to full training on Monday, missed the majority of last season after rupturing a cruciate knee ligament in the Champions League defeat to FC Basel in December.

There are also doubts over the fitness of fellow defenders Jonny Evans and Phil Jones, who are both struggling with foot injuries, while Chris Smalling has been ruled out until October after undergoing surgery on a damaged metatarsal last week.

"It's not 100% Evans and Vidic will start the season. Jonny will start running today. Vida (Vidic) and Phil Jones started full training today," Ferguson said.

Ferguson was also cautious on setting a return date for midfielder Darren Fletcher.

The Scotland captain was forced to take an extended break from the game in December after being diagnosed with ulcerative colitis, a rare bowel condition.

"We don't expect him back immediately, but hopefully he does come back at some point," Ferguson said.