Ralf Rangnick says Manchester United's wingers were told to attack Brighton's centre-backs in the second-half in a tactic that resulted in the dismissal of Lewis Dunk.
United led through Cristiano Ronaldo's 51st-minute howitzer and Lewis Dunk was sent off straight from the restart when he was dispossessed by Anthony Elanga and brought the teenager down.
The VAR advised the on-pitch referee Peter Bankes to review his initial decision to yellow card Dunk, who was sent off after Bankes watched a replay on the pitchside monitor.
Elanga started the game on the right-hand side and Jadon Sancho on the left but the pair switched at the start of the second-half and Elanga's pressure on Dunk forced the error.
Brighton were the superior side in the first-half when United switched to a 4-2-3-1 formation midway through and Rangnick was reluctant to abandon the system during his half-time team talk.
"Of course, we told them at half-time, the question was do we change formation or do we change the way we attack?" Rangnick explained. "And we decided to stick to our 4-2-3-1 and told our wingers, Anthony Elanga and Jadon Sancho, on the ball side to attack the centre-backs.
"And that meant our full-backs had to jump to their full-backs, which paid off when we scored the goal but also the situation with Anthony Elanga and the red card from Dunk.
"And from then on it was a different game, we should have killed off the game much earlier. It should have been 3-0 or 4-0 and again we were not quite lethal enough in those moments to kill the game off and in the last five minutes it's always dangerous against 10 men and you can concede a goal. We took the game to bed with Bruno [Fernandes]'s goal."
Right wing-back Tariq Lamptey had such a profound impact off the bench for Brighton Rangnick replaced Sancho with left-back Alex Telles in front of Luke Shaw, who was booked for cynically fouling Lamptey.
"Their right side with Lamptey was the dangerous side," Rangnick noted. "Lamptey played up front, almost like a right winger for 15 minutes, and that's why we decided to bring on Alex Telles to support Luke Shaw on that side."
