Current optical networks, while offering outstanding reliability, still suffer from occasional failures. A resource-efficient procedure to handle these failures in un-protected scenarios is to perform restoration, i.e., to dynamically setup a backup lightpath after the primary one stops working, which leads to traffic losses while such operation completes. In this paper we propose a technique, applicable to optical networks with centralized control, to better handle failures with slow transients. The idea is to proactively perform the backup lightpath's setup, triggered by either a fixed or an adaptive threshold. The latter is chosen so as to balance the need of offsetting the setup time with the need of preventing unnecessary setups. Simulations show that the adaptive threshold provides better performance than the fixed one, in terms of both timely restorations and unnecessary setup operations.