We're looking at around six to eight weeks from now.Garry Monk
And the Blues boss is hopeful that if there are no further setbacks the 24-year-old striker could be back in contention in as little as six weeks.
Vassell has been sidelined since last October after damaging the ACL in knee and was just weeks away from a return when he picked up the thigh problem last month.
"He's (Vassell) had to have a small operation on the top of his thigh but that's gone really well," said Monk.
"We're looking at around six to eight weeks from now.
"There was a little bit of a tear in the top of the thigh muscle away from the bone but that's all been reconnected by the specialist."