Team stats, change from last season to this season:
ORB%: improved slightly from 10th to 5th
DRB%: plummeted from 5th to 19th
https://www.basketball-reference.com/leagues/NBA_2018.html
https://www.basketball-reference.com/leagues/NBA_2019.html
(see Miscellaneous Stats section)
Ed Davis vs. Zach Collins, rank on team
Davis last season: 1st in ORB%, 1st in DRB%
Collins this season: 6th in ORB%, 9th in DRB%
https://www.basketball-reference.com/teams/POR/2018.html
https://www.basketball-reference.com/teams/POR/2019.html
(see Advanced section)
Analysis: To compensate for the loss of Davis, the brave combined effort of all Blazers except Collins has resulted in our moving upward 9 places in ORB% and 4 places in DRB%. Collins' contribution is to move us downward 4 places in ORB% and 18 places in DRB%. Thus the net changes are (+9 - 4 =) 5 places upward in ORB% and (+4 - 18 =) 14 places downward in DRB%.
Thus, our rebounding would have been higher this season with Ed Davis instead of the Collins calamity.