Socceroo Rhys Williams suffers Asian Cup setback
Williams, who missed the 2014 World Cup after suffering a ruptured Achilles in January, was expected to be ready to return to Boro pre-season training last week.
However, the Perth-born defender, who turns 26 next week, was limited to riding a bicycle at Boro training last week.
Boro boss Aitor Karanka conceded they would err on the side of caution with his return.
The Spaniard hinted Williams faces missing the remainder of the calendar year, effectively ruling him out of January's Asian Cup.
“I prefer to think about January or February and if they are back in November or December that’s better for me,” Karanka told the Evening Gazette, speaking about Williams and Mustapha Carayol.
Williams has endured an injury-cursed career to date, missing the 2010 World Cup due to a pelvic problem, while he battled an ankle injury in late 2012.
Last week, Karanka had said he hoped Williams could be fit for the start of the Championship season which gets underway for Boro on August 9 at home to Birmingham City.