Buying a LB is going to be tricky because there's every chance Mendy comes back in 12 weeks time and doesn't get a serious injury for...years.
Look at Aguero, he's had 1 voluntary surgery and a car crash in about 4 years. Before that he was getting injured every couple of months. Or you look at Delph who couldn't put a month of fitness together in his first 2 years here and then has missed about 5 games since the start of 2017.
Zinchenko & Delph probably go too if we buy a starting quality LB because they aren't going to play with De Jong or another midfield back up to Fernandinho going.
It's not a straightforward move at all.