The Red Devils were heavily linked with several defensive targets prior to the last deadline passing.
They were ultimately left frustrated in that search, with no additions made to an area of their side which has sprung an unfortunate leak in the 2018-19 campaign.
Leicester star Harry Maguire was considered to have been their first choice, having starred for England at World Cup 2018, while Tottenham’s Toby Alderweireld has seen a switch to Old Trafford mooted for some time.
Both are understood to have been considered by Mourinho, but his sights were also trained on Serie A as he sought to address supposed weaknesses by landing proven performers.
Ex-England boss Capello told Sky Sport Italia: “I met Mourinho during a meeting between managers last summer.
“He wanted to sign two defenders, one was Milan Skriniar, the other one was Koulibaly.”
With no fresh faces brought in, Mourinho has had to make do with those already at his disposal.
Chris Smalling, Phil Jones, Eric Bailly and Victor Lindelof have all seen game time, but a settled centre-half pairing is yet to be found.
United even experimented with Scotland international midfielder Scott McTominay in a three-man defensive unit during a trip to West Ham, but found themselves on the wrong end of a 3-1 scoreline.
It now remains to be seen whether Mourinho will still be at the helm by the time the next window opens.
His future at Old Trafford continues to be called into question, with various figures connected with United having had their say on whether the Portuguese should stay or go.
Paul Scholes has suggested that the Red Devils are being “embarrassed” by their current boss, but Ryan Giggs thinks he should be given time and Paul Ince believes underperforming players should be shouldering more of the blame.