This is what Mourinho said about Darmian's role earlier in the season:-
Jose Mourinho has explained
Matteo Darmian‘s role at Manchester United, noting that the Italian’s more traditional style balances well with
Antonio Valencia‘s attacking approach at right back.
The 27-year-old, with
Luke Shaw injured, has started at left-back on nine occasions during Man United’s sixteen game unbeaten run.
He featured on the left flank during United’s 1-1 draw and provided some much needed solidity before being replaced for Maroaune Fellaini with 15 minutes remaining.
Mourinho, when asked about the nature of Darmian’s role by an Italian journalist,
quipped: “Do you want to buy him? Are you an agent? Do you work for the Italian federation?”
“When you have
Antonio Valencia playing as a right winger you cannot defend with two players, you need that third player to give some cover inside and to keep some balances, and he [Darmian] did that well."
Sounds to me like he following his manager's instructions.