Rashford scored a sensational second-half hat-trick after coming on in the 63rd minute.
The Group H match was on a knife-edge at 1-0 when the boss made the first of five substitutions just after the hour.
United turned on the heat and swamped the Germans with Rashford getting three, Mason Greenwood getting the ball rolling in the first half and Anthony Martial getting off the mark for the season from the spot.
“The manager told us when we came on to up the tempo and, for us, that means going forward and there were definitely spaces there and we managed to find the passes,” Marcus told BT Sport.