The striker was withdrawn in the 61st minute of Wednesday night's 4-1 defeat.
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Slot said afterwards: "Especially at the start of the second half, I constantly felt, 'What's wrong with Hugo?' It felt as if he couldn't move as he did the first half. So, that's why our press was, I think, also a little bit off.
"And he just told me that in the first five to 10 minutes of the first half he already felt his back too much.
"That's also why he had to go out."