  Scattering and production amplitudes involving scalar resonances are known,
according to Watson's theorem, to share the same phase $\delta(s)$. We show
that, at low energies, the production amplitude is fully determined by the
combination of $\delta(s)$ with another phase $\omega(s)$, which describes
intermediate two-meson propagation and is theoretically unambiguous. Our main
result is a simple and almost model independent expression, which generalizes
the usual $K$-matrix unitarization procedure and is suited to be used in
analyses of production data involving scalar resonances.
