Rarely, the whole 'tired legs' thing is overplayed by fans anyhow. It's 2019, professional players, fullbacks especially, are incredibly fit and capable of doing intense sprints up to the 90th minute of most games.
Obviously fresh legs helps and is a bonus, but he's not going to suddenly be murdering a fullback just because he's come on late, or all PL clubs would be utilising that tactic all of the time. And the quality on the end of his runs has to be questioned considering his pretty average output at championship level, where I'm going to say the athleticism and ability of defenders is generally much lower. It's mostly just hope and the fact he's a new, exciting new signing driving these narratives, which I don't blame people for .. I've fallen victim to it myself at times with many players.
He's a talent but I'd really be surprised if he had any major impact first season, good time to bed him in/get some games in the cups and use him in a sub some games. He's a 21 year old coming from a lower league.