Why did Bellerin come off at half-time?

Hector Bellerin could not continue after the first half of our 2-2 draw with Crystal Palace on Sunday.
 
So what’s the latest news on our right back?
 
“Yes, he’s injured,” said Unai Emery. “I hope it’s not a big injury, but he had muscular pains.
 
“I don’t know [whether he’s a doubt for the Liverpool game next Saturday].”