I didn't understand his selection and Rooney's omission. I think Welbeck did a good job defensively against Madrid but I didn't think he did very well offensively, other than running a bit quicker than the Real defenders. I didn't see him do anything that Rooney couldn't have done and Rooney offers more attacking options imho, he's a more dynamic player.
Exactly the reason as you said. Welbeck is quicker and can run the channels better so when it comes to the counter attack which United were set up for, he's the better choice.