Fagan has already been told he will be out for three months, following the horror tackle by Newcastle’s Danny Guthrie just over two weeks ago.
Tigers boss Phil Brown said: “Craig has had to have an operation.
“We put a camera into his ankle because there was a suspected rupture of a ligament.
“Fortunately, the surgery showed that the damage was not as severe as we first feared.
“Obviously, the downside is that he has still got a broken leg, and now the expected return date for Craig will probably be three to four weeks later than we had hoped for.
“So we are probably looking at him being back on the training ground four to six weeks after his initial three-month lay off.”
Fagan joined Hull from Derby last month for £750,000.
