The USA captain broke down during Monday night's 4-4 classic at Manchester United.
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"Tyler is injured and has torn his MCL, so he will definitely be out for some time," Iraola confirmed, speaking ahead of Saturday's game at home to Burnley.
"From our previous experience with MCL injuries, it is normally around two or three months.
"It's a big blow because he is an important player for us. Straight away, when you see the mechanism of the injury and the action itself, we felt it could be something serious."
The Cherries boss added: "Tyler is very valuable for us and very unique in what he does. We do have options - Alex has been playing there and Lewis Cook is in his natural position there - but it's true that Tyler is difficult to replace."