  Using a new impurity density matrix renormalization group scheme, we
establish a reliable picture of how the low lying energy levels of a $S=1$
Heisenberg antiferromagnetic chain change {\it quantitatively} upon bond
doping. A new impurity state gradually occurs in the Haldane gap as $J' < J$,
while it appears only if $J'/J>\gamma_c$ with $1/\gamma_c=0.708$ as $J'>J$. The
system is non-perturbative as $1\leq J'/J\leq\gamma_c$. This explains the
appearance of a new state in the Haldane gap in a recent experiment on
Y$_{2-x}$Ca$_x$BaNiO$_5$ [J.F. DiTusa, et al., Phys. Rev. Lett. 73 1857(1994)].
