The Red Devils played out a 0-0 draw at Anfield to remain top of the table - for now.
Solskjaer brought off Fernandes in injury time against the Reds for Mason Greenwood, and was visibly upset on the bench after the decision.
Asked about the reasons for substituting Fernandes, Solskjaer told the club's website: "It was a tactical decision.
"Bruno has been running his socks off for us in every single game since he came here. Running, more or less, every game.
"Towards the end, I felt Paul's physical presence in there might be a chance for us.
"Mason [Greenwood], you know if he gets a chance he can score in the blink of a second.
"But it was a throw of the dice to get Mason on, even though he had to defend against Robertson."