A group within the first-team squad are becoming disillusioned after a 10th defeat of the season at St James' Park on Saturday.
A number of players remain behind Ten Hag and his staff, but there are concerns within the club that the mood inside the dressing room will dip even further unless there is an upturn in form soon. United face crucial games against Chelsea, Bayern Munich and Liverpool in the next two weeks.
Among the issues being raised privately among some players are Ten Hag's intense training sessions since the preseason tour of the United States, and what are considered to be cavalier tactics which have left the team open, particularly during the 3-3 draw against Galatasaray and the defeat at Newcastle.
Ten Hag, according to a source, has tried to cool the atmosphere within the squad by backing his players in public despite some poor individual performances.
According to a source, there is a feeling held by some members of staff that certain players do not respond well to criticism, particularly comments made in public.
It was noteworthy, according to one source, that Lisandro Martínez, a staunch supporter of Ten Hag, has been back around the first team group despite being sidelined with injury.
Martinez is recovering from a second operation to correct a foot problem and is not due to return to action until after Christmas, but the Argentina defender has been pictured watching training at Carrington.
One source has told ESPN that Ten Hag has missed the influence of Martinez and Casemiro in the dressing room while the pair have been injured. Casemiro is also expected to be out until after Christmas while he recovers from a hamstring problem.
Off the field, a source has told ESPN that Sir Jim Ratcliffe's proposed investment is unlikely to be confirmed this week.
Ratcliffe and INEOS, his chemical company, are set to buy a 25% stake in the club while also taking influence over the sporting side of the business.
Sources have told ESPN that there's growing frustration at the time it's taking for Ratcliffe's investment to be confirmed and the uncertainty it's causing around the futures of staff currently at the centre of the football department.
Meanwhile, a source has told ESPN that United were upset with referee Robert Jones' decision not to send off Fabian Schär during the defeat to Newcastle.
Ten Hag and captain Bruno Fernandes, according to a source, visited Jones in his room after the game to ask for an explanation following a challenge on Fernandes by Schär in the second half.