The search for therapeutic drugs that can neutralize the effects of COVID-2019 (SARS-CoV-2) infection is the main focus of current research. The coronavirus main protease (Mpro) is an attractive target for anti-coronavirus drug design. Further, α-ketoamide is proved to be very effective as a reversible covalent-inhibitor against cysteine proteases. Herein, we report on the non-covalent to the covalent adduct formation mechanism of α‑ketoamide-based inhibitor with the enzyme active site amino acids by QM/SQM model (QM= quantum mechanical, SQM= semi-empirical QM). To uncover the mechanism, we focused on two approaches: a concerted and a stepwise fashion. The concerted pathway proceeds via deprotonation of the thiol of cysteine (here, Cys145 SgH) and simultaneous reversible nucleophilic attack of sulfur onto the α-ketoamide warhead. In this work, we propose three plausible concerted pathways. On the contrary, in a traditional two-stage pathway, the first step is proton transfer from Cys145 SgH to His41 Nd forming an ion pair, and consecutively, in the second step, the thiolate ion attacks the a-keto group to form a thiohemiketal. In this reaction, we find that the stability of the tetrahedral intermediate oxyanion/hydroxyl hole plays an important role. Moreover, as the α-keto group has two faces Si or Re for the nucleophilic attack, we considered both possibilities of attack leading to S- and R-thiohemiketal. We computed the structural, electronic, and energetic parameters of all stationary points including transition states via ONIOM methodology at B3LYP/6-31G(d):PM6 level. Furthermore, to get more accurate results, we also calculated the single-point dispersion-corrected energy profile by using ωB97X-D/6-31G(d,p):PM6 level. Additionally, to characterize covalent, weak noncovalent interaction (NCI) and hydrogen-bonds, we applied NCI-reduced density gradient (NCI-RDG) methods along with Bader’s Quantum Theory of Atoms-in-Molecules (QTAIM) and natural bonding orbital (NBO) analysis.