The first part of the paper is inspired by a theorem of H. Rosenthal, that if an operator on $L_1[0,1]$ satisfies the assumption that for each measurable set $A \subseteq [0,1]$ the restriction $T \bigl|_{L_1(A)}$ is not an isomorphic embedding, then the operator is narrow. (Here $L_1(A) = \bigl\{x \in L_1: \,\, {\rm supp} \, x \subseteq A \bigr\}$.) This leads to a natural question of finding mildest possible assumptions for operators on a given space $X$, which will imply that the operator is narrow. We find a partial answer to this question for operators on $L_p(0,1)$ with $1

This paper has been withdrawn, since a new better proof of one of the main results has been communicated to us. We will edit the paper to include this improvement and add a coauthor to the paper. We will resubmit the paper after these improvements have been made