Rooney told Sky Sports: "It was a high getting to the semi-final in Europe but it was a blow to lose Marcos and Zlatan, two very important players for us and we are all gutted for those two players. We wish them a speedy recovery but that gives other players a chance to come in and try to stake a claim for a place in the team.
"The lads who have come in, myself and Anthony, we’ve given the manager something to think about.
"We’ve come in, played well and both scored. It is great for the team to have competition for places and that is what we need from now until the end of the season."
Rooney became United's record goalscorer earlier this term, but has seen his game time sharply reduced under Mourinho.
And he revealed his frustration at seeing his chances limited further by injury.
"You want to play football and I liked being back out there today," added Rooney. "It was important for the team to win today and set up a big game on Thursday, but of course to score today - it’s always great to be on the scoresheet.
"I’ve had injuries and you want to play of course. Just as I am getting my getting chances coming off the bench I’ve picked up a few injuries which has been frustrating. Nothing big, but small injuries which have set me back in terms of choices for the team.
"I’ve come back, I feel strong and thankful to play today and to score."
United will be without Ibrahimovic, Rojo and Juan Mata at the Etihad Stadium, but Rooney insists United will be up for the occasion.
He said: "It’s two good teams, great teams and it is a massive game for the city as always, there is a fight for the top four and us and City are both in that fight. It is a big game for both of us and we’ll be ready."