United were floundering at the start of the season but four wins in five games have steadied the ship for a beleaguered Ten Hag, who was named the Premier League's manager of the month for November when they had a 100% record.
Defender Maguire had been heavily criticised and was on the verge of a club exit in the close season when he fell down the pecking order before he opted to stay and fight for his place.
He eventually got back in due to injuries to Ten Hag's first-choice defenders and the England centre back helped United keep three clean sheets to win the Player of the Month gong.
"We have a squad and there's internal competition. If you want to have a successful season you need more than 11 players. It will change throughout the season," Ten Hag told reporters ahead of Saturday's home game against Bournemouth.
"Scott and Harry played themselves into the team. That's the dedication you expect from every player."
McTominay is the club's top scorer with five league goals, outscoring all United's forwards combined, and Ten Hag said he was not surprised by the Scotland international's eye for goal.
"From the first moment I saw Scott, he has a very good smell to get in the box and he's a very good finisher," he said.
"That's not a surprise. When we get the balls there, he will finish."
On the other hand, England forward Marcus Rashford has netted only twice in this campaign despite scoring 30 times for United in all competitions last season.
Rashford was dropped from the starting lineup against Chelsea but Ten Hag is confident the 26-year-old will return to form soon.
"Rashford is an incredibly good player. You can't do it with 11 players and he can't play every game. He's not in the form he was last year but I'm sure he will get there," he said.
"It's up to the players (to get back into the team). The team will line themselves up and the best players who form the best team will play."
