Manchester United manager Louis van Gaal has confirmed the club are interested in signing at least one new defender before the end of the summer transfer window.
Luke Shaw and Ander Herrera have already joined the club this summer and Van Gaal has said he wants to assess his squad before making decisions about reinforcing other areas of his squad.
But following the friendly wins over LA Galaxy and Inter Milan on penalties, Van Gaal has come to the conclusion he must add to his back four, which has already lost Nemanja Vidic, Rio Ferdinand and Patrice Evra this summer.
"I am looking at the defenders we have first," he said.
"And, when you see our three matches, we are doing well, I think. But it is right that we are looking for defenders, because a lot of defenders have gone.
"We have to replace them but I also like Michael Keane and Tyler Blackett."
Vidic and Evra have moved to Serie A sides Inter and Juventus respectively while Ferdinand has joined newly promoted Queens Park Rangers ahead of the new Premier League season.
The triple departure has left United with a relatively inexperienced group of defenders, with the likes of Chris Smalling, Phil Jones, Jonny Evans and Rafael set to begin the season as first choice in the club's back four.
Despite seemingly lacking depth at the back, Van Gaal was impressed with a second successive clean sheet of their United States tour against Inter.
"I am very pleased," said van Gaal. "I think we played by far the best match (of the tour).
"We deserved the victory I think. I am pleased, not only because of the way we kept possession, but because of the quality of the opponent.
"They had only one shot on our goal and that was given by the referee (a free-kick).
"We also created six or seven chances, so then it is seven against one."
Van Gaal has been linked with bringing in Netherlands defender Ron Vlaar and Stefan de Viji after the pair were part of a defence which conceded just three goals in seven World Cup matches in Brazil.