Chelsea will face Manchester City in the final next month. It ended a run of five consecutive Premier League defeats and gave the club a lifeline after one of the most turbulent weeks in recent memory.
McFarlane had no hesitation in handing Fernández the armband with Reece James still injured.
"He's a winner, he's got so much talent, he's got so much fight, he's massive for this group. And the best thing about Enzo is that he can do a bit of everything, but when it gets tough, you see the fight in him, you see him driving the group on, you see him making tackles, you see him fighting for every loose ball. So he was exceptional today and deserved his Man of the Match."
On the captaincy decision: "I'm very aware that I'm the interim coach, and Enzo is the vice-captain of this club, so I wasn't hesitant to make him captain. I said that to him at the end, I said, 'You like scoring at the back post for me'. And he just laughed."
McFarlane also explained why the result was about more than a trip to Wembley.
"It was important to break the momentum and the form that we were in. I thought we were confident that we would do that today, and I think that completely changes the feel within the group. I think that was probably more important and gives us confidence going into the next five games."
Fernández had been dropped earlier in the month after his flirtation with Real Madrid and received no public support from teammates after the Rosenior sacking. Sunday's display represented a significant personal response to a difficult period.