Ole Gunnar Solskjaer has fired a warning to Edinson Cavani to always be ready after his boot mix up on the weekend.
The Manchester United striker was named on the bench against Southampton on Sunday, and was brought on in the second half with his team trailing.
But Cavani, who was told to get himself ready to replace Mason Greenwood in the changing room, had a problem with his boots.
This resulted in United having to play with 10 men for the opening minute while the Uruguay international struggled to change his footwear on the sideline.
Speaking about the incident, Solskjaer said: "He probably wasn't able to warm up on the pitch [at half-time] but then suddenly he realised he needed bigger studs and I'm happy he did.
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"I don't know he might have to learn on that one as a sub you always have to be ready. I know more than anyone about that."
Cavani's short delay was worth it from a Manchester United point of view however, with the forward turning the match on its head.
Bruno Fernandes reduced the deficit for the Red Devils before Cavani headed home United's equaliser a quarter of an hour before the end.
With their tails up, United rallied, and in injury time Marcus Rashford picked out Cavani once more, who completed the turnaround with another headed effort.
The win means ninth-place United are now two points behind Leicester City in fourth, who were beaten 2-1 by Fulham on Monday night.