Valencia went down after a seemingly innocuous incident involving Kirk Broadfoot and immediately signalled he was in trouble.

TV replays showed his foot pointing in the wrong direction and he was eventually stretchered off, with Broadfoot in obvious distress.

"It is a dislocation or a fracture and he gets operated on tomorrow morning," confirmed Ferguson following the 0-0 draw.

"It was an innocuous challenge but when you saw the Rangers player waving, you knew it was a bad one.

"It is a bad blow for the boy. It is similar to the injury Alan Smith got and you are talking about the rest of the season."

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Arguably, Smith was never the same player for United after he suffered his injury in a freak accident against Liverpool in 2006.

At the time, Ferguson described it as the worst injury he had ever seen.

In total, Smith was out of action for seven months, which if repeated in Valencia's case, would keep the winger out until mid-April.

It certainly presents Ferguson with a major problem because without the former Wigan man, he is left with just Nani, veteran Ryan Giggs and untested Gabriel Obertan as orthodox wide-men.

At least Ferguson was able to report good news on Wayne Rooney, who on his return to first-team action went over on his ankle just before half-time and was limping noticeably immediately afterwards.

"Wayne Rooney is OK. He went over on his ankle but it was no problem," said Ferguson.

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Ferguson denied he had taken liberties with his team selection, even though his decision to make 10 changes to the side that drew at Everton on Saturday backfired.

The one mistake he did admit to was not putting in-form Dimitar Berbatov on the bench as cover.

"We wanted to see how Hernandez would do in a full game," said Ferguson.

"Because of that we left Berbatov out and his form has been absolutely fantastic.

"He has been one of our best players this season and his ability to create in tight situations would have made a difference."

In the end, United had nothing more to offer than Darron Gibson's piledrivers, which came close, but not close enough to prising open the blanket Rangers defence.

"It was a frustrating night for us," he said.

"Rangers' system of play was very difficult to break down. I don't know if playing anyone else would have made any difference.

"They were well organised and defended so deep, it was difficult to open them up at all.

"We didn't test their goalkeeper at all, apart from a couple of shots straight at him.

"It was a really difficult night."