Keane watched the match along with Gary Neville, Jamie Carragher and Ian Wright in a live Stick to Football YouTube special
Arsenal reduced Liverpool’s lead at the top of the Premier League to seven points with a crucial against United at the Emirates Stadium.
After Fabian Schar’s late equaliser denied Liverpool victory at St James’ Park, Mikel Arteta’s men were offered the chance to inch closer to the summit.
And the Gunners did not disappoint with Jurrien Timber and William Saliba both scoring from corners in the second half to provide the hosts with a win that kept their title hopes alive.
Keane gave his verdict on a Stick To Football special on YouTube, as he suggested the flaws that have undermined United for the last few years will be tough for Amorim to erase.
"You are on about these players, but what are we expecting from them? What have we seen from them for the last three or four years? This is what we have seen," said Keane.
"We've seen them do well in the cup but in the league, this is what we’ve seen.
“What was I expecting? For Man United to go and win at Arsenal? No, but I was expecting them to have a go and if they get beat 2-1 or 3-1, but they must be sitting in the dressing room afterwards wondering what happened.
"They need to look in the mirror and ask if they gave everything tonight. Arsenal will be looking back saying ‘that wasn't too bad’.
"I’m really disappointed with Manchester United. I thought the players were going through the motions a little bit.
“He’s only in the door, but the manager’s probably thinking he thought United were a bit better than this. I was getting really frustrated at the end there.”
Keane also suggested United’s hope of a top four finish in the Premier League may be forlorn after their latest disappointing display.
"It shows you what a job Amorim has to do there,” he continued. “It’s a massive job. That was obviously their toughest game and they have come up well short.
"When you see them come up against a good team like Arsenal, they are a long way off.
"I can’t see United finishing in the top six. Can you?
"It’s good that he has come in now and he’s taking the hits rather than everyone expecting him to fix it if had a pre-season. It’s a massive job he’s taken on.”
Keane also gave his verdict on the title race on a night when Liverpool’s 3-3 draw at Newcastle meant their lead at the top of the table was trimmed to seven points after wins for Arsenal and Chelsea.
"I’ll stick with Liverpool,” he added. “Obviously they will be kicking themselves to draw, but they scored three goals away from home and they have a big game at Everton at the weekend.”