Goals from Rayan, Marcus Tavernier and Evanilson secured the points for Marco Rose’s side on Sunday. United conceded twice in the first half before Evanilson added a third after the break, leaving Michael Carrick’s team facing further questions about their form.
Speaking to MUTV, Maguire identified the opening goal as particularly damaging, arguing that United had failed to defend a routine set-piece situation properly.
“You can't slip and give a goal away like that. The third goal, yeah the wind had a massive part to play in that but the first goal is the most important one,” he said.
“We don’t defend a basic set play. We can speak about Leny [Yoro] and Senne [Lammens, for the second and third goals] but the most important one is the first goal and the set play is just nowhere near good enough.”
Rayan opened the scoring in the 38th minute, finishing at the far post from Justin Kluivert’s free-kick. Yoro’s slip then allowed Adrien Truffert to set up Tavernier for Bournemouth’s second, while Evanilson took advantage of the conditions to beat Lammens late in the game.
Maguire said Manchester United could not afford to neglect the fundamentals while results were going against them. He also pointed to previous setbacks, including dropped points against Leeds United and Hull City, as evidence that the issue needed urgent attention.
“When things are tough and you're not getting results you've got to dig in, you’ve got to do the basics well,” he said. “Defending set pieces is part and parcel of the game and we've not been doing that well enough this season.”
The defender added: “We’ve got to do more, we've got to improve more. We cost ourselves three points last week at Leeds with a set play, Hull, and then today the most important goal was the first goal. We’ve got to do more.”
United’s next assignment is a Premier League trip to Chelsea, giving the squad little time to dwell on the latest disappointment. Maguire said the players needed to use the week to rebuild confidence and approach the next fixture with a different mindset.
“When you're coming off a result like that you want to play again as soon as possible,” he said. “Hopefully we can have a good week, we can change the mindset, the confidence within the group, the belief and bounce back.”
Despite the poor start, Maguire stressed that the campaign remained recoverable, provided United responded with better performances.
“The only good thing about the start of the season is that it's still early on. There's still so much to play for, so many points to play for, we can have a good season but we've got to improve and do more,” he said.
“As individuals and a team, we have got to do more.”