Targeted protein degradation (TPD) technology, such as proteolysis-targeting chimera (PROTAC), has become a new therapeutic modality. However, the degradation of undruggable proteins, such as those involved in protein-protein interactions (PPIs), using PROTAC is still limited owing to the difficulties in finding small-molecule binders of these proteins. To identify new chemical moieties that bind to the target sites of the protein of interest (POI), we conducted a site-specific and fragment-based covalent ligand screening using liquid chromatography-tandem mass spectrometry (LC-MS/MS). To apply the selected hits to the PROTAC approach, two-dimensional (2D) nuclear magnetic resonance (NMR) experiments were performed to evaluate the reversible binding of their analogs without covalent warheads. To proof the proposed approach, human mouse double minute (MDM)2 was selected as a model system since it is involved in PPIs and is known to be a degradable target protein. Western blot analysis showed that newly synthesized PROTACs, incorporated reversible analogs of screening hits, affected degradation in a dose- and time-dependent manner.