Chris Wilder expects a different proposition from Nigel Pearson's Watford to the team the Blades played earlier in the campaign.
United took a point from the contest at Vicarage Road back in October, despite dominating the ball for long periods.
But the Hornets have since appointed Pearson as boss, with the ex-Leicester manager inspiring his new club to a victory over Manchester United in their last outing.
Wilder admitted the previous encounter was a great 'learning curve' for his side, and expects the lift generated by his opposite number will ensure a really tough game for his side on Boxing Day.
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