United fell in the first game of the new season as Pascal Gross struck twice for the visitors.
Maguire later said: "We went into the game positive, we had a good pre-season and it was the worst possible start to be 2-0 down at half-time at Old Trafford. ot good enough. We got a goal back but we huffed and puffed a bit and it's a bad start for us.
“We started pretty well and on the front foot and as a team we had control. Then we conceded the first goal and that knocked us badly. We need to look at not letting that first goal set us back as much as it did. It was a nightmare start to be 2-0 down at half-time.
“We spoke about getting the next goal [at half-time]. I think in the second half you saw good parts to our game but overall we didn't do enough. We had a lot of pressure and a lot of ball but we need to be a bit more creative and we need to improve on that
“It's something we need to address or it'll keep happening. The first goal felt like a big blow to all 11 of our players.
“It's great to have Lisandro Martinez at the club. We have competition for places at centre-back. We've not played much together and in that first period we weren't on the same wavelength. Centre-back partnerships are built over time and we will get a lot better and keep clean sheets.
“Everyone knew the task. We had a bad season last season. We've got a new manager trying to put his philosophy on the team but when we don't get it right we look far too open."