How does Jacob get hurt in eclipse?

In the battle between the Cullens + Werewolfs and Victoria's new born army, the Vampires and Werewolfs split the numbers to kill. At the end of the battle, the wolves didn't count how many they killed and didn't realise that they missed one. Leah, a female wolf, found the missing new born and wanted to prove herself by killing it herself. Jacob managed to get Leah out of the way but had no time to defend himself. The rest of the wolves killed the newborn, but not before Jacob got hurt.