The misfiring Red Devils are under intense scrutiny after Sunday's meek 3-0 derby defeat against Manchester City at Old Trafford was compounded with a League Cup loss against Newcastle.
United went out of the fourth round of the Cup 3-0 as well, suffering their eighth defeat in 15 matches and making it the club's worst start to a campaign since 1962-63.
Defiant Dutchman ten Hag said he was a "fighter" immediately afterwards but there have been suggestions his players lack the same desire to dig themselves out of this current hole, starting with Saturday's league trip to Fulham.
"That's not right," the United said when doubts about his players' spirit was put to him. "You can't say that.
"Remember only the fight we showed against Brentford, that (2-1) comeback? If there wasn't spirit in the dressing room, if there wasn't characters in the dressing room, you can't do this.
"I have a good squad and I am convinced of the players in this dressing room.
"These players, we have seen last year, can do a lot better but that is up to me to let them play better.
"I have to take the responsibility to let them play better and I will put every effort in to get this done."
Ten Hag brushed aside talk of January reinforcements as ownership speculation continues, saying United have already "constructed a good squad" before pointing to their injury issues.
Casemiro has joined the list of absentees after sustaining a hamstring injury against Newcastle that United say will keep him out for several weeks.
Lisandro Martinez, Luke Shaw and Tyrell Malacia are among others already sidelined and Victor Lindelof will join them unless he can shake off an illness.
"(Raphael) Varane is ready to play, so he will travel with the squad," ten Hag said after the 2018 World Cup winner missed the Newcastle match through illness.
"Victor Lindelof has some illness. We have to see if he is ready for tomorrow."
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