Essentially any Derabbanan is a Deoraysoh of Lo Sasur.
While that is technically true, it is not applied for practical reasons. If you actually applied that reasoning, then there would never be *anything* that is d’rabbanan. You could never apply the rule of “safek rabbannan l’hakel” since every d’rabbannan is also d’oreisa.
The Wolf
Actually, the Rishonim already ask about why we DON’T say that every Derabbanan is a Daoraysoh in respect to Sofek. I think it is a machlokes Ramban/Rambam if every Derbanan is in fact a Deorayso or not.
R’ Shimon Shkop (Sharey Yosher 1:3) and the Shev Shmaytseh (1:23) bring the various shittos down.
1) The Rabanan were never Goizer in a case of Sofek
2) Being oiver misafek is not included in “Lo Sosur” (For explanation of this, see the Nesivos who takes on that the Rebanan cannot “asser” something; you just have to listen to them. So if you are oiver beshoggeg, for example, you will not be oiver on Lo Sassur. See Kovetz Shiurim — Kuntras Divrei Sofrim.)