Manchester United 1 Arsenal 1 - Giroud late goal wipes smile of Jose's face
Arch rivals Jose Mourinho and Arsene Wenger shook hands at the start and end of a topsy-turvy match which saw the Gunners steal a late equaliser through Olivier Gori=oud.
Late challenges and intelligent defending restricted both teams from open play early on, with set-pieces looking their best bet.
Mata curled a left-footed free-kick wide for United and Alexis Sanchez somehow managed to direct a close-range header away from goal and team-mate Aaron Ramsey's reach.
It was a glaring miss from a player whose movement and endeavour was proving a nuisance, although the Chilean - who had been a doubt after a hamstring complaint - was left on thin ice after a clumsy challenge on Ander Herrera resulted in a yellow card.
Darmian joined him in referee Andre Marriner's notebook as play became edgy and increasingly disjointed, with the United full-back perhaps fortunate not to pick up a second yellow card after catching Carl Jenkinson in the 33rd minute.
The officials were coming under increasing scrutiny as the first half wore to a close, with Mourinho apoplectic after his team's penalty claims were ignored.
Valencia flicked the ball through Nacho Monreal's legs and went down under pressure from the Arsenal defender, but Marriner waved away his appeal to the chagrin of the Old Trafford faithful.
Petr Cech's reactions prevented frustration quickly turning to elation when Mata made space and got away a low shot destined for the bottom corner.
The Arsenal goalkeeper turned that behind and again had to be alert four minutes before the break, tipping away an Anthony Martial attempt before comfortably denying the Frenchman from the resulting corner.
Referee Marriner was jeered as the teams made their way off at the break, with Mourinho's re-emergence for the second half suggesting he learned from United's last home league game when anger at the officials earned a sending-off and one-match touchline ban.
The Portuguese's side restarted brightly as Martial and Michael Carrick had scuffed efforts, and a blocked Marcus Rashford shot drew claims from the Stretford End that a penalty should have been given for handball against Shkodran Mustafi.
As the chances began to dry up, and Phil Jones denied Laurent Koscielny getting a close-range header away, Mourinho plumped for substitute Rooney, with Martial going off and Rashford pushed wide.
It added an extra edge and coolness as United looked to make the breakthrough, with that hard work paying off in the 68th minute.
Pogba played in the overlapping Herrera, whose intelligent cutback was met by a first-time strike by Mata to send the crowd wild.
The Spaniard was booked for celebrating with the crowd, before Marcos Rojo's powerful header went close to doubling United's lead.
Toothless Arsenal looked set for defeat, only to catch United cold in the dying stages.
Alex Oxlade-Chamberlain darted all too easily past Rashford and sent in a cross that Giroud converted to shock Old Trafford.