Bristol decided to prioritise its coming Premiership Cup game, which was not on the agenda when the Reds match was organised, and Lam said he was left with no alternative.
"We have to qualify from the Premiership Cup; otherwise, we'll be left for four weeks without a game ahead of our league fixture against Exeter, which is not acceptable.
"We had only one training session for this game, so I'm pretty proud of the effort we put in."
Bristol was only able to compete in scrums and lineouts. The Reds took a full toll in the loose, showing far more speed and shredding the Bristol defence. With first five-eighths Tom Lynagh in complete control, the Reds ran in 11 tries to Bristol's four, including one to Northland and former Crusaders wing Heremaia Murray on his first appearance for the Reds.
Reds coach Les Kiss was pleased with the way the side gelled.
"We had about eight or nine new combinations in our team, so for those things to click well for us was. We had good, accurate passing, good clean outs, good breakdowns; all the little things that you want to do well."
The Reds will play their second game in Ulster on Friday.
The Waratahs and Brumbies played out a 36-36 draw in Bowral.
The Waratahs had some injury concerns, deciding not to play Joseph-Aukuso Suaalii and having captain and halfback Jake Gordon leave the field early in the game with a back injury.
Andrew Kellaway made an impression on the wing for the Waratahs, the side he last played for in 2018. After a stint overseas, he returned to play five seasons for the now-disbanded Rebels in Melbourne.
The Brumbies demonstrated depth behind Noah Lolesio, who didn't play at first five-eighths, with Declan Meredith shining and scoring a fine second-half try. Australian Under-20s second five-eighths Kadin Pritchard also showed why he is regarded as a long-term prospect for the side with a classy display.
The Force played an internal game in Perth and will play the Brumbies in their only warm-up game against other sides, which will be against the Brumbies on Friday in Canberra.