For the long standing low mass puzzle of $Lambda_c(2940)^+$, we propose an unquenched picture. Our calculation explicitly shows that the mass of the $Lambda_c(2P,3/2^-)$ state can be lowered down to be consistent with the experimental data of $Lambda_c(2940)^+$ by introducing the $D^*N$ channel contribution. Additionally, we give a semi-quantitative analysis to illustrate why the $Lambda_c(2940)^+$ state has a narrow width. It means that the low mass puzzle of $Lambda_c(2940)^+$ can be solved. What is more important is that we predict a mass inversion relation for the $2P$ $Lambda_{c}^+$ states, i.e., the $Lambda_c(2P,1/2^-)$ state is higher than the $Lambda_c(2P,3/2^-)$, which is totally different from the result of conventional quenched quark model. It provides a criterion to test such an unquenched scenario for $Lambda_c(2940)^+$. We expect the future experimental progress from the LHCb and Belle II.
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