Maguire, whose abuse from supporters had former player David Beckham reach out to him in support, headed home in the 72nd minute, while Onana saved a penalty in the final seconds of the match for United's first Champions League win of the season.
Ten Hag said defender Maguire, who was also excellent in Saturday's 2-1 Premier League win at Sheffield United, is playing much more proactively in possession.
"Stepping in passing vertical, defending also on front foot, stepping in defending forward, very confident in the duels, he is dominating, dominating his opponents," Ten Hag said.
"And then you see also a very good skill from him with his heading, great pass from Christian (Eriksen) as well but very good finish."
Onana has been harshly criticized since arriving from Inter Milan in the transfer window. There was nothing but love for the goalkeeper on Tuesday, however, after he dived left to push away Jordan Larsson's spot kick and was mobbed by his team mates.
"He showed personality, and he knows that before was not the levels that his skills are, he didn't match his skills and he could do better and I think Saturday (at Sheffield) was a very good performance and today as well," Ten Hag told reporters.
The victory, which began with a stirring tribute to club great Bobby Charlton, who died on Saturday at age 86, propelled United into third in Group A on three points after three games.
While United stumbled to a horrible start to their domestic season, they have won their last three games in all competitions - perfect timing for a hot streak, Ten Hag said, ahead of Sunday's derby against Manchester City at Old Trafford.
"There is a great spirit and they are together and that is important always as a team, the team is more than the sum of the individuals," Ten Hag said. "That is what we have to take into the Manchester derby...to fight together and then we have a very good chance to get a result."
Ten Hag, who has called Charlton a "leader" and a "giant," Charlton's former teammate Alex Stepney, and Under 19s team captain Dan Gore followed a lone piper playing "The Red Flag" on to the pitch.
Charlton, an England World Cup winner and one of Manchester United's greatest players, died on Saturday at the age of 86.
"We love you Sir Bobby... a hero to millions, the finest English footballer the world has ever seen. You will live on in our hearts forever," the public address announcer said, before asking for a minute's silence.
Supporters then broke into "There's only one Bobby Charlton!"
Hours before kick-off, fans flocked to Old Trafford's Trinity Statue of Charlton, George Best and Denis Law -- the trio of former United greats - waiting in line to carefully add bouquets and scarves to the hundreds already encircling the statue's base.
The words "Sir Bobby Charlton, 1937-2023, Forever Loved," had been added over Old Trafford's main entrance, a wreath was placed in Charlton's seat inside the stadium and the players wore black armbands for the game.
Charlton was a key figure in England's 1966 World Cup-winning team, playing alongside his brother Jack, and he made 758 appearances for United, scoring 249 goals.
