The 26-year-old had started the Premier League season in brilliant form but was cruelly struck down with an ACL injury in his left knee during last weekend’s loss to Newcastle.
Speaking ahead of Saturday’s clash at the Gtech Stadium against Everton, Frank said: “It is a big loss for us, he is a key player, there is no doubt about that.
"Mostly I feel for him. I think he has done so well so far this season.
"In my opinion, I’m not the head coach of the national team of England, but he must have been very close to being picked for Euro 2024 - that is, of course, out of the question now.
"Unfortunately, that is part of football. We will find a solution and make sure we put 11 good players out there and a good team out there this weekend."
Aaron Hickey is expected to move over to left-back to cover Henry’s absence, while Mads Roerslev and Kristoffer Ajer are both available for the clash with the Toffees.