Here is a quick summary of his stats compared to some top players who distribute & Hernanes & Lemina.
A few things to consider:
1) Not all of these players have identical roles. I think all do play the distributor role for their teams but some have more attacking freedom than others and some have more defensive duties than others. Some are occasionally used as explicitly CAM or DM too.
2) The sample size for Lemina and Hernanes is very small. So the real comparison is between witsel and other top players. I used 2014/15 for Verratti because of his injury last year.
3) Witsel plays in a much weaker league but I do not know whether that affects passing stats the way it affects goals/assists.
4) I used league figures since every one of those plays for a dominating team in their league, while Axel and Moutinho play for weak teams in CL.
5) I never watched him play. So if he is the distributor of Zenit then I am very underwhelmed by this signing and would rather go for Bastian.