Daniels boss Graham Drury bemoaned three big decisions going against his side as Stamford were knocked out of the FA Cup on Friday night.
Armstrong’s dismissal was the first of three decisions which irked Drury who felt his side should also have had a late penalty while play should also have been stopped due to an injury to Jake Duffy in the build up to City’s winner.
Drury said: “It’s very frustrating because three big decisions have gone against us.
“Mike has won the ball, but caught the lad with his follow through. The referee said he sent him off for intent.
“Jake has got a big scrape down his shin. He’s down, but the referee doesn’t stop the game and they score from the next attack.
“Sam Hollis was also in the penalty area when he was fouled late on. I was gobsmacked when a free-kick was awarded on the edge on the box.”
Stamford did shade the dominance of a close contest but, despite enjoying large parts of possession, they failed to create a real clear-cut chance against a well-drilled City side.
Drury added: “We were patient with the ball, moved it from side to side, but we didn’t create enough opportunities to score.
“They defended well but, apart from several scrambles and a few blocked shots, we didn’t cause them enough problems which was disappointing.”
Stamford made just one change to their starting line up following Tuesday night’s league victory over Carlton Town with Henry Eze handed his first start after an impressive substitutes performance in midweek.
The Daniels made the most of the early running with Joe Burgess heading narrowly wide from a Ben George cross.
Burgess again tested home goalkeeper Niall Conroy from distance before Stamford custodian Dan Haystead did well to hold an effort from Lewis Wilson.
Stamford lost frontman Elliot Sandy to injury midway through the first half and clear-cut chances continued to be at a premium with Duffy fizzing in an effort which Conroy held at the second attempt while Haystead made a decent block at the other end to deny Tom Knowles.
Conroy did well at the start of the second half to deny Burgess with a smart stop before Duffy and Eliot Puttnam combined to cross for Jordan Smith whose header was superbly tipped to safety by the home keeper.
The turning point came when Armstrong was dismissed midway through the second half as the Lilywhites took control of the contest with Haystead producing a fine save to deny Salim Relizani.
Home goalkeeper Conroy also had to be at his best to keep out a Rob Morgan effort before Cambridge got the crucial goal with just six minutes remaining as Relizani rose highest to head home a cross.
Stamford piled forward in search of an equaliser and thought they had been awarded an injury-time penalty when Hollis was felled, but a free-kick was awarded on the edge of the box and Duffy’s effort was headed off the line by Mason Spence as Cambridge clung on for victory and ensure there will be no repeats of last season’s cup heroics from Stamford.
Daniels: Haystead, George, Puttnam, Armstrong, Gordon, Eze, Morgan, Duffy, Smith, Sandy, Burgess. Subs used: Hollis, King. Not used: Marshall, Hill, Jeys, Challinor, Draper. Attendance: 236.