McNamara has been punished with an immediate three-game suspension by a Scottish Football Association disciplinary panel with two games suspended for his third breach of rule 203 which covers abuse of a match official.
McNamara was sent to the stand with Tommy Wright, manager of St Johnstone, during the Perth side's 1-0 victory at Tannadice on March 12.
Wright, who admitted his first offence, was punished with a one-game ban that has been suspended so he can be on the touchline at Ibrox on Sunday when his side takes on Aberdeen in the other semi-final.
McNamara was found guilty of misconduct by the "repeated use of offensive, insulting and abusive language" while Wright was found guilty of breaching the same rule by "leaving the technical area in the absence of special circumstances and by adopting a threatening and aggressive attitude towards an opposing member of team staff".
McNamara's suspended ban lasts until the end of next season, while Wright's ban runs out at the end of 2014.
McNamara was unable to attend the hearing at Hampden because it clashed with his team's training session and the SFA would not delay proceedings until later in the afternoon.
A statement from United read: "The club is disappointed at this outcome, particularly as the manager was unable to attend the hearing in person, and will now consider whether to appeal."
McNamara said: "I am obviously disappointed with this decision. I will be discussing the matter with the club and we will consider whether or not to appeal against it."
He added: "I would have liked to have attended the hearing in person and asked for it to be held over until the afternoon. As it was, I felt it was impossible for me as Dundee United manager to miss our training two days ahead of a Scottish Cup semi-final."
Any appeal by United would not be heard until after the semi-final and this would both allow McNamara to take his customary place for the club's biggest match of the season and court SFA sanctions for a course of action that could be regarded as "frivolous".
Earlier, McNamara had been questioned at United's training session in St Andrews about the hearing.
"I saw Tommy on Monday night at the Hibs game," he said. "I had a chat with him and there are no problems." He also stressed the importance of discipline at what could be a physical encounter at Ibrox tomorrow.
McNamara is aware that Ally McCoist, his counterpart at Rangers, is under pressure after defeat by Raith Rovers in the Ramsdens Cup final on Sunday.
But he said of the criticism directed at the Rangers manager: "I don't think it is justified but that is the nature of the beast when you manage at the Old Firm."
McNamara, who played for Celtic for 10 years from 1995, added: "You have millions of people looking to praise you or give you dog's abuse. As a club they were similar to Celtic when they lost to Raith Rovers and it is the embarrassment that goes with it or the disappointment and everyone jumps on that."
He said: "Ally is used to that as a player and manager but he knows if he can turn it around this weekend then that will be the most important one, even more so than last week, and that is what we have to be wary of."