The 19-year-old is the only uncapped player in the 25-man squad announced on Tuesday.
Defender Aaron Hughes and striker Jamie Ward are included but are injury doubts for the 14 November game in Bucharest.
McNair's Old Trafford team-mate Jonny Evans is out injured and has yet to feature in the qualifying campaign.
He missed the first qualifying game in Hungary because of suspension and an ankle injury ruled him out of the victories over the Faroe Islands and Greece.
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Northern Ireland have made a magnificent start to the qualifying series, winning their opening three games for the first time.
They are two points ahead of the second-placed Romanians.
McNair, who has featured three times for United this season including Saturday's derby defeat by Manchester City, has been brought in as cover for Hughes who picked up an ankle injury playing for Brighton.
"I've been hugely impressed with Paddy - he's come into a difficult situation at Manchester United and there is a lot of media attention," said Northern Ireland boss Michael O'Neill.
"The manager has come in and spent a lot of money so it is encouraging to see the likes of Paddy given a chance.
"He's played in difficult circumstances, with the defence a bit makeshift at times and it's difficult for a young player to come into that.
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"But he's acquitted himself very well, he's around the Manchester United first team and I'm looking forward to having him in the squad."
West Brom midfielder Chris Brunt returns to the squad after missing the games against Greece and the Faroe Islands, but Hull defender Alex Bruce has a hamstring injury.
Northern Ireland squad: Goalkeepers: R Carroll (Notts County), A Mannus (St Johnstone), M McGovern (Hamilton Academical).
Defenders: A Hughes (Brighton), C Baird (West Bromwich Albion), G McAuley (West Bromwich Albion), R McGivern (Port Vale), C Cathcart (Watford), C McLaughlin (Fleetwood Town), L McCullough (Doncaster Rovers), R McLaughlin (Liverpool), P McNair (Manchester United).
Midfielders: S Davis (Southampton), C Brunt (West Bromwich Albion), S Clingan (Kilmarnock), C Evans (Blackburn Rovers), O Norwood (Reading), B Reeves (MK Dons), P McCourt (Brighton), N McGinn (Aberdeen).
Strikers: K Lafferty (Norwich City), J Ward (Derby County), J Magennis (Kilmarnock), B McKay (Inverness Caledonian Thistle), Will Grigg (MK Dons).
Manchester United defender Paddy McNair has been named in the Northern Ireland squad for the first time for the Euro 2016 qualifier away to Romania.