I am guessing that in the next few days Klopp will have to have a difficult talk with Sakho to tell him he will not be the starting CB for the season. There are several subtle clues to this - the way the club distanced itself from Sakho's PED investigation and trial (I would think they would be more vocal if someone they considered a key player was caught up in this), Klopp's always complimentary talk about Lovren and the rumors of him being given the captaincy, even the way the gaffer answered a question about Sakho last night: I would say that the hard part is over, but I need to talk to Mama first...
One phrase that really stuck with me from Klopp's first interview arter he was appointed, was that in football, just like in life, you need to "build from the back", to ensure you have a solid foundation before anything else. It's clearly a fundamental belief for Klopp that transcends football, so he is certainly not likely to go into a season with any kind of sense of uncertainty about his first-choice defensive partnership. In his time in Dortmund one of his first orders of business was to settle on a new CB partnership of Hummels-Subotic which remained unchallenged throughout his time there (it helped that they were fucking great). He did have a very capable 3rd choice CB in Felipe Santana (who many considered to be the best 3rd choice CB in the Bundesliga) and he would always perform well whenever one of the first-choice pair was out injured - the question is whether Sakho is OK with such a role.
It's clear that Klopp wasn't at all satisfied with the state of our central defense when he came in - that's why the contract of Matip was negotiated already in January. I also don't think that he completely trusts Lovren yet - maybe that's why we've been monitoring a number of young, potentially top class CBs this summer - but over the course of last season Lovren has been the more consistent of all our players in his position, so it makes perfect sense that Klopp would select him over Sakho or anybody else to partner Matip, at least until some new young superstar defender can be found.
Obviously all of this is just reading the tea leaves at this point and it remains to be seen if things pan out in the way I'm predicting. But I would be surprised if Matip-Lovren is not the starting CB partnership next season.