"To weaken the side" means to undermine, reduce its strength or capability, or make it less effective in achieving its goals or objectives. Full definition
We played a grossly weakened side for the cup and they played a full strength squad. (anfieldindex.com)
Pochettino has fielded weakened sides in FA Cup ties at Newport County and Rochdale, and now at Brighton, and the outcome has been the same: underwhelming draws. (espn.com)
This is weakening the side of the marginal communities in the conflicts between commercial interests and survival needs» -LRB-» Ecological Perspective of Development» in Development and Politics of Survival, pp. 71 f). (religion-online.org)