Norris went off the track at Stavelot in the first half of a wet Q1, skating through the gravel as his race engineer suggested the Briton had sustained front wing damage.
Closer inspection in the garage revealed Norris had significantly damaged his floor, which left him compromised for the rest of the mixed-weather session.
Norris ended up making it into Q3 but had to settle for seventh behind rookie team-mate Piastri, as the McLarens struggled for top speed down the straights.
"I went off in Q1 and completely destroyed the floor, so I'm happy to have made it to Q3 and only be a couple of tenths off what Oscar did," Norris told Sky Sports F1.
"I think he did a good job. His middle sector was very strong. We were very poor in the straights.
"There's wins and losses, but I'm happy. It could have been a lot worse, I could have been out in Q1, so I'll take P7."
When asked to explain how much of his MCL60's floor was damaged, Norris said: "The whole thing. Normally, you can repair it [under parc ferme rules].
"Tape is not going to help. We tried to tape it. They did a good job on getting as much as they can fixed, but too much to be competitive today.
"So, I don't think we're bad. I think if we had a car that was together, if I hadn't made the mistake in Q1, we could have had maybe a slightly better day.
"We weren't miles off the guys ahead, but I'm happy I got what I did out of it."
Piastri also felt he hadn't extracted the maximum performance out of his McLaren.
Source: Autosport