Video: Sergio Busquets falls like a sack of potatoes after being hit by a lump of grass
Jeremy Mathieu scored a late headed goal to give Barcelona all three points from a testing encounter at Celta Vigo.
The French defender outmuscled his marker at the far post to head home Xavi's free-kick in the 75th minute, just two weeks after nodding in the opening goal in the 2-1 win over Real Madrid.
Barcelona struggled for large parts of the game against coach Luis Enrique's former employers -who were reduced to 10 men in the closing minutes as Fabian Orellana was shown a straight red card for throwing a piece of turf at Sergio Busquets - but they stay four points clear of Madrid at the top of the Primera Division thanks to Mathieu's header.
Here is the incident which saw Orellano red-carded:
Celta were the first team to beat Barcelona at the Nou Camp this season and they nearly took the lead after 10 minutes here, with Claudio Bravo diving across his goal to keep out a low, driven effort from Joaquin Larrivey, the man who scored the only goal back in the game in November.
Lionel Messi produced Barcelona's best moment of the first half, teasing his way past several Celta players on the edge of the area before firing at Sergio Alvarez. The goalkeeper did well to parry the Argentinian's shot although Neymar should have turned in the rebound, but could not keep his shot under the crossbar.
Celta appealed for a penalty on the half hour mark when Claudio Bravo came out to take the ball off Nolito, but the referee ruled that the Chilean had got to the ball before clashing with the forward.
Nolito and Messi both missed the target early in the second half but Dani Alves was left looking far more embarrassed when he scuffed a clear chance to score at the far post. Then Celta striker Charles raced towards goal but Gerard Pique produced a perfectly timed challenge to rob the ball from the Brazilian before he could shoot at Bravo.
With Celta successfully blocking Barca's attempts to score from open play, Barca eventually found a way through from a set-piece. Substitute Xavi swung the dead ball to the back post and Mathieu escaped the grasp of marker Andreu Fontas and stretched out to head the ball under the bar from a couple of yards out.
Messi should then have wrapped the game up but chipped over the bar with just Alvarez to beat and then Orellana was given his marching orders for his bizarre confrontation with Busquets.
The hosts kept pushing for an equaliser until the final whistle, but could not find a way through.