It's so hard to gauge the value of Sixers players. Often times the numbers are inflated since someone has to take the shots/get the rebounds even if the game is a blow out or the opponent isn't trying since the Sixers are so bad.
For example two years ago I said that Henry Sims isn't an NBA player. Several people on this board said I was being ridiculous because he was putting up solid NBA numbers. Well, he's no longer in the league and is just one of many examples of Sixers players being overrated because of their numbers.
As to Covington, I think I would trade them Zeller and a second or two for him. Rozier might not be putting up good numbers now, but he probably has more long term value/upside. I'm sure people will strongly disagree with this, just like they strongly disagreed me saying Henry Sims was over valued (Covington is obviously way more valuable than Sims but the idea of him being overrated due to increased numbers he wouldn't get on other teams is valid)
The reason I would trade for Covington is because we are weak at his position. Brad Stevens likes to break down positions into four groups, ball handlers, wings, swings and big men. Covington is a swing and really the only other swings we have are Jerebko and Crowder.