If he is bought to be a leftback, then he would have to invert to suit our playstyle.
That being said, I still don't want anymore inverting fullbacks like Cancelo or Lewis.
The CB pushing into midfield that Stones does was a masterstroke from Pep, as it gave him 5 Tall Defenders, like:
Rodri, Stones
Ake, Dias, Walker
That is very good at stopping counter attacks and winning aerial and ground duels, freeing up the 5 attackers ahead to have more freedom and "try things".
Back to Guerreiro, if he is bought, I'd use him more as the 8/10 role like Gundogan.