We already took out Coquelin for Giroud.
So we were left with Ozil and Xhaka with capability to play CM.
If we need to chase, we need Ozil upfront with Sanchez.
So if we bring in Lucas, Xhaka has to handle the midfield with Sanchez, Lucas, Walcott, Ozil and Giroud in advanced position.
When we can't handle City with two midfielders, it doesn't make sense with just one. Will only open us for more goals.
Its pretty logical substitution but don't let rationality come into picture.