VORP stands Value Over Replacement Player and it's a statistic that attempts to determine how many points better/worse a player is per 100 possessions than a replacement-level player. By using lots of averages and measures, it normalizes for a league-average team in the process. VORP is not a bullet-proof stat. None of them are. VORP has Jahlil Okafor as being -0.8 last year and being essentially a replacement-level player. This is one of the stats that "trade Jah" folks are leaning on.
Among players with an average of 20+ minutes on the court, Okafor had the 2nd worst Net rating among centers in the league.
Net rating essentially tells the point differential between team and opponent, while a player is on the court. For Jahlil, that's not good.
The theory was that he would make the team better and they'd play closer to their opponents when he was on the floor. The statistics say otherwise. With Okafor on the floor, the Sixers averaged 92 points per 100 possessions. Their opponents averaged 108 points per 100 possessions.
You would expect that any player on the Sixers would have a negative Net rating because the team was so terrible, but -16 is insane. Nerlens Noel's Net rating was -11, despite playing for the same terrible team and occasionally sharing the floor with Jahlil. Okafor's was the worst on the team by several points. In fact, the entire team's season-long Net rating was -10.4. Not a great sign for Okfaor.