Sir Alex Ferguson has indicated Wes Brown will miss the remainder of the season.
Brown has managed just two substitute appearances since last October after sustaining a foot injury but seemed to be heading for full fitness again.
But Ferguson said: "Wes had a little setback during the week. It is not major but the fact the season is coming to an end makes it difficult for him."
Ferguson was hoping to introduce Brown back into the first-team picture at Middlesbrough last week but in the end had to rule him out.
Although the damage is not thought to be extensive, with only four matches of the Premier League season to play, plus the Champions League final as United look to retain a trophy Brown helped them to win in Moscow 12 months ago, Ferguson has effectively written the 29-year-old out of his plans.
Instead, the Scot will rely on John O'Shea, Rafael Da Silva and Gary Neville, who is fit again after his own foot injury, to fill the right-back berth.
Ferguson added: "We have Gary back fit so we are not too bad. But it is disappointing for Wes."
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