Despite boasting a 4-1 lead from last week’s last-16 first leg at Old Trafford, the Red Devils boss only made four changes for the return fixture from Sunday’s Premier League draw with Southampton.
Betis made life difficult for United during the first half at a bouncing Estadio Benito Villamarin, which was silenced in the 55th minute when Rashford rifled home in stunning fashion.
The 25-year-old emerged from the break with the bit between his teeth and thumped in a 25-yard effort just minutes after he had uncharacteristically blazed over a glorious chance.
“The team was creating some chances,” Ten Hag said. “Rashy had some chances before and he missed some but he kept his belief, he kept trying, and got his reward.
“All good strikers know that not every chance you will get a goal but at this level, you don’t create five or six chances in a game so one of the two has to be in.”
Rashford’s 25th United goal in major European competitions was his 19th in 24 matches since returning from the World Cup.
“(His form) tells you something about the team’s performance,” Ten Hag said. “It tells you something about him.
“If he keeps the focus, and makes sure he has the energy, the way, and the way he has to play in that system and style, he will get in scoring positions and then it’s about him, he has the ability to finish.”
Ten Hag had always suspected United would have to ride out an early storm on Thursday and was happy with the way his side eventually took control in the second half.
Bruno Fernandes and Casemiro also managed to play a substantial part without picking up bookings that would rule them out of the first leg of their quarter-final tie.
The latter’s four-match domestic ban made his starting role against Betis less surprising than Fernandes, who has played in all but three of United’s games this season.
“He is very strong, very good recovery,” Ten Hag said. “He is a very good athlete, he has so much energy, and he can play in so many roles.
“Tactically he is brilliant, he can play in roles and we can benefit from him, we can overload areas, we can outplay opponents, we create good situations and create chances.
“He is very important in the counter press, and pressing or transitions. He is a good example for the team.
“In recent weeks, he has been in brilliant form and he looks like he is getting better from game to game.”
Fernandes was joined in the line-up in Spain by Facundo Pellistri, who made his first United start since joining the club from Penarol in October 2020.
“I think he deserved to come in,” Ten Hag said of the 21-year-old Uruguay international, who had only made six substitute appearances before Thursday.
“He had a good training performance, you see him progressing, getting stronger, good game against Leeds, against Betis.
“To start is more difficult, we have seen that, but when the game goes on he came in and got belief, you see his abilities.
“He had some good dribbles, good runs in behind, and some good crosses, defending he did his job, so I am pretty pleased with his performance.
“For him it is a step up. Now it’s up to him, to get rhythm and contribute to the team.”
As for Betis, all focus is on domestic action after former Manchester City boss Manuel Pellegrini oversaw a 5-1 aggregate defeat in this last-16 tie.
“When we drew Man Utd we knew we would be in for a difficult match,” the LaLiga side’s boss said.
“They are a Champions League team playing in the Europa League. We had three good chances to score but I am proud of this team for the fight they showed.”