United went forward en masse for an early corner and kept nobody back to guard against their Turkish hosts' counter-attacking. As soon as United lost the ball, Edin Visca pumped a long ball upfield from inside his own penalty area and Demba Ba was able to run unchecked from inside his own half before beating Dean Henderson.
"That's completely down to organisation," Scholes said. "At a corner you surely always have someone back on the halfway line."
Speaking on BT Sport, former United midfield general Scholes pointed to assistant manager Mike Phelan telling Nemanja Matic that he should have been covering the possible break.
"It's like under-10s football," Scholes said. "You can understand it if it's the last minute and you're going for a goal."
This took place in the 12th minute, however, and for Scholes there was no excuse.
"It's embarrassing," Scholes said. "What the back four were doing, I've no idea. Defensively they've got to take responsibility. You've got to defend one-v-ones at times at Manchester United and they seem incapable of doing it."
Fellow former United midfielder Owen Hargreaves was equally baffled by the blunder from Ole Gunnar Solskjaer's side.
"I'm not sure I've ever seen a goal like it," Hargreaves said. "The problem is the players are waiting for a cross that doesn't really materialise and they never really get back. One ball up the pitch and Demba Ba's in on his own."
Basaksehir went on to win the match 2-1 and claim their first ever victory in Champions League football.
United, whose good form in Europe has masked their weak start to the Premier League season, remain on six points in Group H. They had previously impressed in a 2-1 victory at PSG on matchday one before thrashing RB Leipzig 5-0 last week.
Ole Gunnar Solskjaer's men will aim to make amends for this surprise loss in a fortnight's time - following the international break - when they host Basaksehir at Old Trafford in the reverse of Wednesday night's fixture.