The opportunities for MU and Barcelona to qualify for the round of 16 are still equally open. However, the Red Devils certainly have advantages that might be very beneficial for them, namely, the advantage of away goals and playing at home. Apart from that, Erik Ten Hag also greatly benefited from the opponent's condition. Barcelona came to Old Trafford with a limp strength. Xavi cannot rely on Dembele and Pedri who are injured, while Gavi is suspended from playing due to accumulation of yellow cards.
That is clearly an advantage for Manchester United because of the condition of Barcelona's players who are having problems with injuries and accumulation of yellow cards, but Manchester United doesn't have to be happy or arrogant about this because Manchester United also still needs to prove itself is the best in front of its own fans. Erik Ten Hag will also instruct his troops not to make mistakes in the second leg at home this week.
Even though Barcelona are not favored in the second leg, I'm sure Xavi will instruct his troops to appear tough until the end of time. This season's Europa League is a competition worth fighting for by United and Barcelona. This second tier event in Europe can also be a springboard for the two teams to return to the top of success.
Barcelona will still be favored by some people and also by their own fans in the second leg later, because as you said, the chances for Barcelona and Manchester United to qualify for the last 16 are still open. And that's despite the advantage Manchester United have at home which will probably be a very good advantage for Manchester United.