Manchester United boss Sir Alex Ferguson admits his players must be at their best tomorrow.

Ferguson says he could not discount the emotion of the Hillsborough tribute having a negative effect on his players.

Liverpool intend to mark the occasion with a number of significant gestures, including a mosaic across three sides of the stadium.

And having gone through something very similar four years ago on the 50th anniversary of the Munich air disaster, the Red Devils boss accepts it may have a hidden impact.

"It's a possibility, I don't deny that," Ferguson said. "Human nature can be that way.

"When we played Manchester City for the 50th anniversary of Munich the place was so flat in the dressing room before the game. I even felt it myself.

"We just couldn't perform and were glad to get it out the way. It was such an emotional day for us and it could be that way on Sunday."