United, who have three points from two games, put in unconvincing displays in their 1-0 win over Wolverhampton Wanderers and 2-0 defeat to Tottenham Hotspur.
Pundit and former United skipper Gary Neville said Ten Hag's decision to play Mount and Fernandes in advanced roles had left too much defensive work for Casemiro, who has also received criticism for a perceived lack of fitness.
Asked if their midfield setup was causing problems for United, Ten Hag told reporters: "You always have to analyse clinically and that assessment isn't right.
"It was about the back and the front. That is why we were so open. Nothing to do with the midfield. We've already said straight after the game.
"If you see his numbers, you'll see Casemiro is fully fit. I don't have any doubts (the midfielders) can do it."
United have been dealt two injury blows in the last week, with Mount and Luke Shaw both being sidelined.
The club's big-money signing Rasmus Hojlund has also been unable to make his debut due to a back injury, with the Danish striker having missed their defeat to Tottenham and win over Wolves.
"Injuries are always disappointing but we can deal with it. Rasmus Hojlund is not available for tomorrow, he came into team training for the first time," Ten Hag said.
"Next week we expect him to do a full week, so he is very near to the squad."
Shaw's injury has complicated Ten Hag's team selection at left back, with back-up defender Tyrell Malacia also sidelined due to an injury sustained last season, but the Dutch manager said the club had planned for such a situation.
"(Mason Mount and Luke Shaw) will be absent for some period. Luke will be longer than Mason. Tyrell Malacia will take a few weeks, we have the solutions in our squad. Definitely," Ten Hag said.
On Monday, United decided to part ways with Mason Greenwood following an investigation into allegations of attempted rape, with charges against the forward having been dropped in February.
Greenwood said in a statement to British media that he did not do the things he was accused of.
Ten Hag declined to comment on Greenwood, saying: "We are where we want to be as a team.
"I have a lot of work to focus on the team; so I'm focussed on the players I have available."
