* United held to 0-0 draw at Real Sociedad
* Substitute Van Persie twice denied by post
* English champions remain top of Group A (Adds details, quotes)
SAN SEBASTIAN, Spain, Nov 5 (Reuters) - Robin van Persie twice hit the post as Manchester United missed a chance to all-but seal their place in the Champions League last 16 when they were held to a 0-0 draw at La Liga side Real Sociedad in Group A on Tuesday.
The Netherlands forward almost scored moments after replacing Wayne Rooney just after the hour at Sociedad's Anoeta stadium in San Sebastian when his far-post shot cannoned away off the right-hand upright.
United were awarded a penalty for what looked like a blatant Ashley Young dive in the 69th minute but Van Persie's powerful strike hit the same post and bounced away to safety.
Sociedad came closest to scoring when a Xabi Prieto effort was deflected just over the bar by Carlos Vela 15 minutes from time before United midfielder Marouane Fellaini was sent off in the 90th minute after picking up a second yellow card.
"We should have won, we had enough chances," United manager David Moyes said on UEFA.com.
"We controlled the game and were the team on top for the majority," added the Scotsman.
"We had a good chance to go clear in the group tonight and take three points and we should have.
"You need to take your chances and opportunities when they come but we are still in a good position."
The English champions stayed top of the group on eight points from four of six matches, with Bundesliga side Bayer Leverkusen on seven and Shakhtar Donetsk on five after they drew 0-0 in Ukraine.
Sociedad are bottom on one point and are on the verge of elimination in their first foray into Europe's elite club competition in a decade.
"We will try and win the group, we have got two tough games but we know what they are all about and we are confident we can go through," said United midfielder Ryan Giggs, making his 137th Champions League appearance for United.
"We should have won but we are happy with the performance and happy with the point," added the 39-year-old Welshman, who made his United debut in March 1991 before several of the Sociedad players were born.
Moyes initially deployed Javier Hernandez up front alongside Rooney, with Giggs pulling the strings in midfield, and the visitors dominated the early stages without creating much genuine danger.
Sociedad started to settle as halftime approached and fullback Alberto de la Bella curled a low effort just past the post in the 45th minute.
Hernandez fluffed one of the clearest openings of the game a minute after the break when the Mexican shanked his effort over the crossbar from close range with Sociedad keeper Claudio Bravo floundering.
Rooney and Hernandez toiled for an hour before Moyes made a double substitution, bringing on Van Persie and Young, and United seemed to find the cutting edge they were lacking.
After Van Persie's first near miss, the referee pointed to the spot when Young tumbled to the ground despite there being little contact with Sociedad captain Markel Bergara.
Both sets of players looked weary in the latter stages and ultimately a draw was a fair result.
"The team tried their hardest for the whole 90 minutes but it wasn't to be," Vela said in an interview with Spanish television broadcaster Canal Plus.
"It was vital for us to get a win here but I think we came up against a very strong side," added the Mexican.
"It was a very close contest and both teams had chances to win and we have to keep fighting now and see what we can do.
"I think the fans enjoyed themselves and the team battled hard for them."
United will have a chance to seal their place in the knockout round when they play at Leverkusen on Nov. 27. Sociedad play at Shakhtar in a must-win game for the Basque club. (Reporting by Iain Rogers in Madrid, editing by Ed Osmond)