set someone back on their heels

set (one) back on (one's) heels

To put someone into a state of surprise, unease, or bemusement, especially such that it affects their ability to perform or function. The sudden arrest of one of our teammates last night certainly set us back on our heels, and we weren't able to play to the best of our abilities this morning. The tragedy set the entire community back on their heels.
See also: back, heel, on, set

set (or rock) someone back on their heels

astonish or discomfit someone.
See also: back, heel, on, set, someone