Rashford interfered – Akanji
City defenders tracked Rashford's run from an offside position as he chased the ball for 30 metres before Bruno Fernandes stepped in at the last second to score.
The linesman raised his flag for offside, but referee Stuart Attwell awarded the goal, allowing United to mount a comeback and win 2-1.
"Fernandes tells him he's not in an offside position and he can shoot. That's when he stopped and for me it's a clear interference," Akanji told Viaplay.
"I understand he didn't touch the ball. But what I always thought the rule was if you chase the ball and your intention is to play the ball... it's clearly offside, but the ref decided it's not."
United's Luke Shaw told Sky Sports that the City defender covering Rashford was not able to reach the ball.
"Rashy obviously made the run but I think the player who was with him, I don't think was going to be able to get the ball anyway," he said. "I don't think he actually did interfere at all.
"I think Rashy was clever to know that Bruno was there and leave the ball. For me, it was a goal."
Former English referees chief Keith Hackett said there was "no doubt" Rashford was interfering with an opponent.
"To allow Bruno Fernandes' goal to stand is total nonsense. If we do not call that offside, then the offside law is an ass," he wrote in The Telegraph.
"They will argue that he [Rashford] has to touch the ball to be active. The law is awful and requires a complete rewrite."