With seven matches to play, City, chasing their first top flight title since 1968, moved on to 71 points, two behind United who would go five clear at the top if they beat lowly Blackburn Rovers at Ewood Park on Monday.
Arsenal's seven-match winning league run ended when lost 2-1 at strugglers Queens Park Rangers while Chelsea maintained their push for a top four spot with a 4-2 win at Aston Villa.
"They had a lot of chances, we made some mistakes and that's it - we can do nothing at the moment," he told Sky Sports.
"We have been really unlucky. We lost two or three important players and we didn't play well, but at least at the end we scored twice and got a point."
Sunderland ended City's perfect home record of 15 straight league wins this season, having taken the lead through Sebastian Larsson after 31 minutes with a shot from the edge of the box.
Lowly QPR, battling against an immediate return to the Championship, were too good for Arsenal at Loftus Road, going ahead through Adel Taarabt's first Premier League goal with a drilled shot after 22 minutes.
Samba Diakite scored QPR's winner with a first time shot - his first goal for the club - leaving Arsenal third in the table on 58 points.
"We made mistakes and our performance was not good enough," said Arsenal manager Arsene Wenger. "We lacked the extra gear, and when you lack that you don't win games."
QPR boss Mark Hughes told Sky Sports: "We showed a lot of discipline and resilience to see the game out. We had a fantastic night here recently when we beat Liverpool but today was even better."
