The Portuguese winger had a procedure in Amsterdam carried out
by a Dutch surgeon, but the club say they will not be able to state
when he will be fit again until he is seen by the specialist in a
month.
It is likely he will be out for at least six weeks and miss the
start of the new season.
United said in a statement: "Cristiano Ronaldo underwent surgery
today to his right ankle.
"The operation was performed by Professor C. Niek van Dijk at
The Academic Medical Centre in Amsterdam and the initial
indications are that the procedure was successful.
"Cristiano will begin his convalescence under the direction of
club medical staff and an estimation of his return to full fitness
will be possible following review by the specialist in one
month."
Chelsea will face Man U in the 5th match in the 08/09
season.