The Croatian international was sent home by manager Steve McClaren to rest, but he may still link-up with his country's squad for their friendly against Eire next week, the club have confirmed.
"It is not thought the injury is serious and the 30-year-old is expected to be available for the opening game of the Premiership season at home to Arsenal a week on Saturday, indeed he is likely still to keep his midweek international appointment," said the club`s official website.
Meanwhile, Boro enjoyed another pre-season win with a 1-0 victory over Eredivisie side Willem II.
New signing Jonathan Greening enjoyed his debut for Boro by setting up the only goal of the game for Dean Windass after 42 minutes.
Greening and England Under 21 team-mate Mark Wilson, who came on after 58 minutes, only sealed their moves to Teesside a matter of hours before the game.
