Callahan saw action for Rosa Radom (Poland-PLK) in 2016-17. He played eight games for them in the Basketball Champions League where he averaged 11.4 ppg and 3.5 apg. Callahan also logged nine games in the domestic Polish league and there he managed to put together averages of 12.8 ppg and 2.1 apg. He later moved to Parma Basket Perm (Russia-VTB) early in 2017. There, he played eight games and recorded averages of 5.6 ppg nd 2.6 apg.