The Red Devils are languishing in 16th place after losing two of their first three Premier League matches of the new season.
And United have already conceded 11 times in the league, with only West Brom (13) conceding more than them this season.
United boss Ole Gunnar Solskjaer is desperate to find a solid centre-back pairing after Harry Maguire, Victor Lindelof and Eric Bailly have struggled with consistency.
According to ESPN, United are weighing up the possibility of signing at least one central defender when the window reopens in January.
The publication reports that Sevilla's Jules Kounde and Villarreal's Pau Torres have been added to United's transfer watch list.
ESPN claims that the Red Devils will monitor both Kounde and Torres from now until the end of 2020 before making potential moves for them.
Kounde, 21, was heavily linked with United rivals Manchester City in the summer as Pep Guardiola overhauled his defensive options.
City eventually ruled out a move for the Sevilla defender and instead signed Ruben Dias and Nathan Ake in the window.
Sevilla value Kounde between £58m and £63m, while Villarreal could allow Torres to leave for around £45m.
United would need to offload some of their current defenders before they sign a new centre-back in the window.
Solskjaer is expected to keep Maguire as his first-choice centre-back, with Lindelof and Bailly also in contention to pair alongside the United skipper.
The United boss could also keep hold of 22-year-old Axel Tuanzebe, who is currently recovering from an injury setback.
However, both Phil Jones and Marcos Rojo are currently on the chopping block after they were left out of United's Champions League squad.
United will return to action on Saturday when they travel away to Newcastle for an 8pm kick-off.
Featured Image Credit: PA
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