Jose Mourinho isn’t enjoying Manchester United’s pre-season campaign. Dealing with an assortment of seniors, reserves, and untested youngsters, the Portuguese manager is reluctantly slouching from city to city in the United States, reaching few conclusions about how his team will look in their Premier League curtain-raiser against Leicester City on August 10.

However, if reports are to be believed, one of the two players Mourinho wants to bring in before the transfer window closes would strengthen the center of his defense. With Eric Bailly and Chris Smalling starting each of United’s two friendlies thus far, Mourinho has at least deduced the need for more in that department.

But who will Mourinho identify as the man to improve his center-back options? Here are five rumored candidates for the Red Devils’ backline.

Toby Alderweireld

Despite being one of the Premier League’s standout defenders in recent years, Alderweireld was the victim of “unjustified” and elongated spells on the bench last season for Tottenham Hotspur. The defender’s relationship with manager Mauricio Pochettino is being tested.

The 29-year-old is also in the final year of his contract with the club, so it’s possible the north Londoners will cash in on the Ajax product. They may have little say in him departing anyway, given his rumored £25-million release clause.

Combining Alderweireld with Bailly would be outstanding. Bailly’s all-around game and willingness to muck in with the dirty work would free up space for Alderweireld to launch balls into the paths of Romelu Lukaku, Marcus Rashford, and United’s other pacey frontmen. Alderweireld’s attacking contributions are equally as important as his defensive offerings.

Jerome Boateng

Bayern Munich chairman Karl-Heinz Rummenigge revealed tentative talks are underway between Boateng’s representatives and Paris Saint-Germain over a transfer. But Manchester United have also been linked to the German.

The 29-year-old, who endured a peculiar one-season spell as Manchester City’s third-choice right-back, has injury problems, but he’s undeniably one of the best center-backs in the world when fit. His positional play is sublime and, like Alderweireld, Boateng is capable of swiftly turning defense into attack by effortlessly brushing the ball with his right leg.

Mourinho isn’t known for his patience with injured players – ask Smalling and Luke Shaw – but if Boateng shakes off his troublesome muscle ailments he could be a shrewd and transformative addition for United.

Harry Maguire

For those who hadn’t appreciated Maguire’s remarkable attributes at Sheffield United, Hull City, and Leicester City, his performances at the World Cup for England were huge wake-up calls.

The 25-year-old is a unique talent, combining old-school physicality and aerial prowess with an ability to surge and instigate attacks as a defender. If the Foxes can retain his services, they’ll have a promising partnership of Maguire and Jonny Evans to begin the season. If they don’t, a rival club will significantly bolster their squad.

The athleticism of a Maguire-Bailly combination would be a joy to watch. But would the notoriously conservative Mourinho allow the Yorkshire lad to gallop out of defense? A purchase of Maguire may require a tweak in approach by the Red Devils.

Kalidou Koulibaly

Koulibaly has been linked with moves from Napoli over the past few summers – most prominently to Chelsea – but he remained under the management of Maurizio Sarri. Now attack-minded tactician Sarri has joined Chelsea, Koulibaly may consider shifting elsewhere rather than sticking around for Carlo Ancelotti’s regime.

He possesses traits that Manchester United wanted from last summer’s signing of Victor Lindelof – good passing, a willingness to step into midfield when required, and clever anticipation – but also provides uncompromising strength and a capacity to head away an asteroid. His header against Juventus last season was a bullet, and temporarily ignited Napoli’s Scudetto hunt.

Koulibaly has three years left on his contract, is entering his peak at age 27, and Ancelotti would prefer to keep him around. His asking price should be high.

Yerry Mina

One of the stars of the World Cup was Colombia’s Mina, who scored three headers in Russia – the most at the tournament since Miroslav Klose notched five during the 2002 edition.

There could be competition for the Barcelona outcast’s signature – Everton and Wolverhampton Wanderers are apparently eyeing the 23-year-old – but Manchester United may be wise to use their financial power and pick up the former Palmeiras player.

Mina has qualities that Mourinho has traditionally wanted from his players, including a 6-foot-4 frame, and he would boost United’s weaponry on set pieces. The Stretford lot’s goal share from headers last season was commendable – it amounted to nearly 18 percent of their league goals – but slotting Mina into a pack also featuring Marouane Fellaini could lift that number closer to Chelsea’s 27-percent share.

The chief concern about Mina, however, is his comfort with the ball at his feet. That perhaps led to his limited opportunities in Catalonia, and was masked by Davinson Sanchez’s strength with the ball on the international stage.