It's a team game and he's playing on the left. If the full back is occupied with the opposition winger then I expect Sigurdsson to be occupying the space around their full back.
Martina shouldn't have followed Son's run into the box, Holgate or Jags should have taken that man and allowed him to go back wide to mark Aurier. He's not a left back, we all know that, so I expect Jagielka to be directing him and Pickford as well.
He's a very limited footballer but that wasn't a result of any of physical or technical deficiencies but a failure in the team holding a solid defensive shape allowing Aurier to occupy that space on the right and that's a team failure as well as Martina's.