Alex Telles would be excellent for you, it isn't as good as Alex Sandro right now but he's definitively one of the best LB currently. His crossing is excellent so at least you would have someone to actually make crosses in your team :P
He won't, of course, be as expensive as Alex Sandro.
The only problem is that we are probably very reluctant to sell him right now, we sold our two best RBs (Ricardo and Dalot, we are left with the 34 year old Maxi Pereira), one of the starting CB left for free (Marcano) and the main sub CB will probably leave too (Reyes) - the other CB sub Osorio has barely played since he arrived and the good loaned CB, Boly, has been bought by Wolves as they triggered the buyout clause.
To make it worse our two u-20 promising CBs (Leite and Queirós) are also being coveted by other teams (Leite by Arsenal).
Our defense right now can only be trusted upon Felipe and Alex Telles, if Telles leaves then it would simply be probably too much to rebuild.
We are under finantial fair play "rule", so any money would be welcome, but giving that we will probably also sell Herrera and Brahimi in this window we can probably hold on Alex Telles.
I did hear the rumour though that Dalot wasn't the only player we were negotiating with United, and from our players Alex Telles would probably be the most needed by you.