Neil Warnock will be without Jonny Howson and Nathan Wood for Saturday's match against Millwall, with the former facing a few weeks on the sidelines.
Wood withdrew from Boro's travelling party ahead of the match against Preston on Wednesday, while midfielder Howson was forced off during it with a hamstring problem.
"Jonny will be out for a while," Warnock confirmed
"We're not sure if its a two-week or a four-week, at his age there is always going to be something shows on a scan, so we’re going to send it for a second opinion.
"He’s had it for a few weeks.
"It’s a blow, but we knew that. He’s one of the major influences on the team. But we have a lot of games coming up and we can’t dwell on it."
Meanwhile, young defender Wood will spend a period away from Rockliffe.
"Nathan Wood has to self-isolate, his girlfriend has contracted the virus.
"It’s another disappointment at a time we don’t need it - he might well have played tomorrow, Woodsy.
"I’ve had it so I can’t pick on anyone, and I've said before how important it is we keep adhering to the rules."
Another absentee is set to step up his recovery at the training ground this morning.
"Anfernee Dijksteel hasn’t sprinted yet, we decided to give him another 24 hours," Warnock said.
"He’ll probably do that this morning.
"He’s not seen any affect normal jogging, but you don’t with hamstrings. We’ve just got to be careful."