The veteran Spaniard had set a time good enough for 13th on his final lap but was vulnerable to progress from drivers further back, culminating in a drop into the bottom five. He couldn't recover, having already pitted, and was out of time to set a final flying run.
Citing the luck of the draw with traffic around the circuit, Alonso lent his support to a suggestion of using group qualifying to set the grid in Monaco - something that junior categories Formula 2 and Formula 3 already employ to minimise the effect of being impeded.
To avoid track conditions favouring one of two groups in F2 and F3 qualifying, the halves are 'zipped' together in position order - and the driver with the fastest lap time across the two groups is awarded pole, handing the leader of the opposite session second on the grid.
"I think that [group qualifying] will be great," Alonso suggested. "I think Monaco is so exciting and so thrilling that it will be nice to see everyone's lap live, because we miss a lot of great moments. But yeah, it's the way it is.
"You can get lucky or unlucky. I think last year I got lucky and I made some progress in Q1, Q2 and then in Q3 you are P2 and maybe some of the big contenders that were out of Q1 or out of Q2.
"This time it was for us the bad side of it and Checo as well. I think he's P18. This is Monaco. You need to find your luck as well and we didn't do today."
Alonso reckoned that he lost three tenths emerging from the Nouvelle Chicane and two more in the Rascasse, which would give him the deficit he needed to make up to reach Q2 at the very least.
The Aston Martin driver did not believe the car could have been in the hunt for pole, and also doubted that an early pit call in Sunday's race would offer much in the way of gains - assuming it follows the same pattern as his early stop at Imola last weekend.
"Obviously we were not maybe the car to be on pole but I think enough to be P7, P8, but I found myself in the wrong place, wrong moment," Alonso reflected.
"This is Monaco, you know it's difficult to get a clean lap, but I think I lost three tenths out of Turn 11 with a car just in front of me , and then another two tenths in the last corner with three cars that were parked there just to start in the lap.
"No one to blame, I think they cannot vanish in the very last moment out of the last corner, but yeah, a little bit unlucky today.
"[On Sunday, the aim is to] not crash, try to keep the car in one piece. Obviously, in this part of the year, we try to bring as many upgrades as possible. So the stock parts are not great. We just need to try to finish the race and get to Canada a little bit more prepared.
"I think it's impossible [to pit early]. I did in lap seven in Imola and in lap eight half of the grid stopped. This is the way it is."
