Mowbray has watched his side concede eight goals in just three Sky Bet Championship games, including three penalties.
The weekend's 3-3 draw against Bournemouth at the Riverside saw Rhys Williams join Emmanuel Ledesma on the list of suspended players after being shown a red card.
And that has proved to be the last straw for the Boro manager, who will spend the week searching for an emergency centre-back with unsettled Norwich defender Dani Ayala a potential target.
Asked about strengthening the backline, Mowbray said: "I think we have to and the players know that.
"The bottom line is the team deserve better than losing three goals, two goals every week to what in my mind is not a tactical thing, individuals have to stop playing their part in costing us goals.
"So maybe we need to create more competition in defensive areas.
"We did try to sign a centre-back in the last window and for a few reasons we didn't get a deal done.
"Maybe this sending off might prompt us to look.
"Let's wait and see what happens. We will look, we will see."