- Hakim Ziyech (29', 62')
- Kerem Aktürkoglu (71')
- Alejandro Garnacho (11')
- Bruno Fernandes (18')
- Scott McTominay (55')
Copenhagen's draw with Bayern Munich later on Wednesday mean United sit a point below Galatasaray and the Danish team with one game to play, while the Bavarians had already qualified prior to Wednesday evening's fixtures.
"It's obviously really bad. We lost really bad goals and had to have control of that situation," United captain Fernandes told TNT Sports.
"We should have taken all three points, clearly," Ten Hag said. "But we should have done that in other games, too. Away to Copenhagen, home to Galatasaray, we play so good.
"But at the same time I have to criticise them as defending we aren't good enough. Winning 3-1, we can't afford to make these mistakes as it makes the difference."
Ten Hag would not point the finger at Andre Onana despite his errors contributing to Galatasaray's goals.
"I think every goal we conceded has its own story," he said. "They come from transition moments and we don't block the middle."
United roared into an early 2-0 lead through Garnacho and Fernandes before Hakim Ziyech pulled one back from a free kick following an error by Onana.
McTominay put United 3-1 up soon after halftime but Ziyech slotted home another free kick under Onana's arm and Muhammed Kerem Akturkoglu equalised for Galatasaray in the 71st minute.
Galatasaray welcomed United to RAMS Park by spelling out "Welcome to Hell" at one end of the stadium and it definitely was for Onana.
"We were on top twice and had far too many chances to close out the game but we weren't clinical enough," Fernandes said.
The game for United was a virtual repeat of their Champions League group clash with Copenhagen in which they gave away a commanding lead. The end-to-end match saw 17 shots for United and 16 for Galatasaray, many in the dying minutes as both sides pushed for victory.
Fernandes had a good chance to score the winner but his 85th-minute effort struck the post.
- José María Sánchez Martínez