Performance studies are made to estimate the ATLAS potential in $B$-physics after upgrade for Run2 and HL-LHC. Real data as well as Monte Carlo simulations are used to study the decay of $B^0_s \to J/\psi\phi$ in order to measure the $CP$ violating mixing phase and the width difference between the $B^0_s$ eigenstates. The increased sensitivity is expected mainly due to the improved decay time resolution obtained with the upgraded IBL and ITk inner tracking detector.