The defender was back to his imperious best against Arsenal in an FA Cup third-round penalty shootout win.
United drew 1-1 after extra time, having played most of the second half and extra time with ten men.
Post-game, Maguire said: “Ruben said to me that when it came across to him that I had an option (in my contract), he said it was a no-brainer for him. It’s given me confidence and belief that they want me at the club and part of this project. He’s really demanding.
“And I’m happy to be here. I want this club to be back fighting for titles and not fighting where we are in the table right now. We’ve got a long way to go.
“I think my first couple of years at United was probably the best part of my career. I’m 31 now, coming up to 32 in a couple of months, but I’m getting back to that level. I also think I was at that level last season as well, so it’s been 18 months now where I’d say I’ve been playing consistently well for this club again.
“There’s no doubt I had a tough year in my third season when Ralf took over, I wasn’t at my best, I knew that, I knew I needed to find something to stay at this club because it demands high standards and I feel like I’ve found something. Mentally and physically I feel good.”