Lukaku has fired in six goals in his last eight appearances.
Jagielka said: "We've never had an issue but people talk. When he's farmed out on loan, there's no big fee, when he's bought, he's exactly the same player, exactly the same person but unfortunately there's a price tag by the side of him.
"As soon as the price tag comes in, people want to have a bit harsher opinion.
"We've paid a lot of money for him, not just for one season but for the future and hopefully he'll come out of the Young Boys game smiling - happy footballers usually play better."
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