Alex Telles I believe will be an obvious upgrade on the left back position for United. He is a very capable defender and is a threat on the offensive end as well both with his good set piece taking ability, crossing, and shooting. He will help solidify your defense for sure,
Telles has been the best left back in the league every season he was at Porto IMO and I think most followers of the leagues opinion. Based on the fact Porto are in financial troubles I have no doubt United got him for a good fee which makes this a no brainer. Telles also has mileage on his legs as by this stage in his career he is experienced for a championship winning team and CL constant.
All in all solid pickup for Manchester United. Will be be a "world class" left back? at 27 already I would say no but I think at the moment he is a starter in this United team and helps improve the team.
Hope this helps mate!