One of the possible reasons why Ronaldo left Juventus is because Juventus failed to get into the Champions League. So playing in the Champions League with Man United was the moment that Ronaldo really wanted, and for that Ronaldo will certainly play all-out.
It will be very interesting if Ronaldo is played as a starter and can play the full 90 minutes but if considering the Young Boys Level is below Man United, and Ronaldo has recently joined Man United, so it is possible that Ronaldo will not be played as a starter (or he will play as a starter but didn't play the full 90 minutes).