Mike Phelan felt Hull City - and Andrew Robertson in particular - were dealt a harsh lesson by Manchester City in their 3-0 Boxing Day defeat.
The Premier League's bottom side Hull frustrated their hosts for 70 minutes at the KCOM Stadium, and even came close to opening the scoring when Michael Dawson's header was cleared off the line by Bacary Sagna.
But with just under 20 minutes remaining, Robertson dived in needlessly on Raheem Sterling and conceded a penalty that was duly dispatched by Yaya Toure.
As Hull's belief crumbled, Kelechi Iheanacho added a quickfire second before Curtis Davies' own goal with the last action of the game completed the home side's misery.
"The penalty was the major turning point," boss Phelan told BBC Sport.
"Andrew Robertson is heartbroken over it because he knows he has suffered a harsh lesson there - he has got to stay on his feet and shuffle the player across into an area where maybe his team-mate can do a little bit better at making an attempt to tackle.
"But he has learned from that, and he will learn a lot more at this level.
"He is a good young player, and a good kid to have around but he has made a basic, basic mistake that has cost us.
"Until then, we put everything together the best way we could. I thought we were very, very solid and moved the ball around well.
"We got to their players quickly and definitely frustrated them, we caused them a few problems now and again.
"This is where we are at the moment. We make a mistake or get caught in certain situations and we need to manage the game a lot better, as professionals to make sure when we are in the game, we stay in the game."
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