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They didn't back down, bumping heads before trading insults which resulted in a booking for both players.
Shortly after their altercation, the Belgium striker's admiration for the veteran Swede reappeared where he spoke about the 'eye-opening' experience at Manchester United.
Lukaku and Ibrahimovic spent a short time together at Old Trafford with the 39-year-old making a big impression on him.
The 27-year-old said everything he learned from the ex-Barcelona and Paris Saint-Germain star was great before recalling a training ground incident.
Speaking to United's official website, he said: "Everything I learned from Zlatan was great, from his stories that he told me when he was at Inter, AC Milan, Barcelona and even Ajax, to being on the field working with him, seeing how competitive he was.
"I remember one training session - because we were both strikers, we were never on the same team - there was a 50/50 challenge. He went full-on against me!
"That is when I knew this guy wants to compete and this guy wants to fight for his spot.
"That is why he changed me. It was an eye-opener. The guy had to fight to be in the position he is in.
"So, I learned just to focus, work hard and enjoy it as well. Because he is a guy who you can have a lot fun with as well and having his personality in the dressing room was good."
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