Grant Hall is facing an extended spell on the sidelines and could undergo surgery.
The summer signing has seen a specialist and could be up to four months out with a thigh injury.
“Grant Hall could be out for 12 to 16 weeks,” said Boro boss Neil Warnock.
“We don’t know if he’ll need an operation or not. We’ve had different opinions so we’ll have to make a decision on what’s best.
“It’s a real blow. He’s worked so hard in training and looked so sharp. I’m absolutely gutted for him.
“It’s not his original injury – it’s a front thigh injury. It was quite innocuous when you look at it. He finished a physical session and just made a side-pass into the goals, but he pulled up holding the front of his thigh.
“The scan is worse than we first thought, but we’ve got different opinions. One surgeon would operate and one wouldn’t – I’m sure they’ll make a decision between them this week.”
With Hall joined by Ashley Fletcher as a long-term absentee and Marcus Browne also still sidelined, the boss says he has one or two further “niggles” to contend with before finalising his squad for Wednesday’s visit of Swansea City.
“We’ll have a look at the lads after training today,” he added. “We’ve over 24 hours before the game so we’ll see how things are looking.”