The 29-year-old new signing has been ruled out for the first few months of the season and is unlikely to return to action before Christmas.

Frustrated manager Steve McClaren said: "It’s a setback, but we must show our resolve. We will need club, players and fans to pull together.

"We’ve got a strong squad that is good enough to do well this season, and Juninho’s misfortune could be an opportunity for someone else.

"With Szilard Nemeth, Alen Boksic, Joseph Desire Job and Noel Whelan, I believe we’ve got cover in abundance."

The World Cup winner rejoined the club for a third time this summer in a £12m (€18.74m) deal from Atletico Madrid.

But it is feared the player may now need an operation before he has even kicked a ball in anger for the club under McClaren’s charge.

McClaren added: "It’s too early to say how long he will be out for, but it’s obviously a serious injury.

"It’s a huge blow not only for the club and the supporters, but for the little man himself.

"After all the excitement of his return to the club and his performance against Alaves last week, this has brought us all back down to earth with a bump."

It is the second major injury setback of Juninho’s career having broken his leg whilst at Atletico and being forced to miss the 1998 World Cup finals.

He recovered from that setback and was back in the Brazil squad that triumphed in the Far East this summer.

Should Boro’s worst fears be confirmed, however, and the player need surgery, McClaren may have to rule him out of his plans for much of the season.