The striker, who has ten goals to his name this season, suffered the injury during their defeat to Crystal Palace at the weekend.
Boro had hoped he would be back in time to face Millwall this Saturday but the problem is worse than first feared and he will be out for at least two weeks.
Jutkiewicz will now miss Boro's FA Cup fifth round clash with Chelsea next week.
"Lukas has ankle ligament damage, he took a heavy challenge on Saturday," said boss Tony Mowbray.
"But I look at Main champing at the bit and I think why wouldn't we play him. He's got physicality and he's scored two goals since coming back.
"We will give Lukas every opportunity to get himself fit and not rush him back.
"Ankle ligaments are delicate things, I've had them myself. I'm aware that we can't keep rushing players back ahead of schedule. Scott McDonald might be ready soon but we'll have to make sure he's ready."