Lionel Messi was left covered in blood after being clattered by Chris Smalling.
The Barcelona forward was smashed in the face by the trailing arm of the Manchester United centre-back after the pair challenged for the ball.
Smalling crashed into the Argentine with his shoulder before flying past him at speed.
His arm then caught the Barcelona star in the face, knocking him to the ground and causing his nose to bleed.
Messi spent some time on the ground as medics came on to offer their assistance.
The Argentine was then patched up and made his way to the touchline before the referee waved him back on after play resumed.
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It doesn't appear that Messi suffered any serious facial damage, despite the gruesome nature of the injury.
Messi had earlier played a key role in Barcelona's opening goal - despite an initial bad touch.
Messi ran onto an Arthur pass and would have been clean through on goal had he got it right.
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But, he more than made amends by flipping an excellent cross onto the head of Luis Suarez, who nodded back and the ball found the net, via the chest of Luke Shaw.
The effort had been initially ruled out for offside, only for a VAR review to confirm that Suarez had been onside from Messi's cross.
