The 24-year-old is a product of Boro's famed academy, but his career has been dogged by injury and he has been limited to just 50 league appearances since his debut in 2007.
Now it has been discovered that Hines has a 'micro-fracture' in his knee, which will need surgery with four months of rehab - meaning he will not be back in training until late July.
"Seb Hines will be going in for an operation over the next few days," said manager Tony Mowbray.
"He might miss the start of the season. It's his knee, he's had a few issues with that.
"He saw a specialist earlier this week and he decided that he needed to go in and tidy it up.
"The diagnosis is that he might miss the start of the season. It's not what we wanted to hear and not what Seb wanted to hear.
"Seb's got a strong mentality and he'll come through it no problem."