Rooney insists the summer arrival from Bayer Leverkusen is battling to adjust to Premier League demands.
Speaking on his Wayne Rooney Show on YouTube, he said: "It's been tough for him. I don't think there's any denying that and he'll be searching deep within himself to fix that and trying to figure it out.
"I think he's played okay and better, but I think when you sign so many players like Liverpool have - and this is not normal for Liverpool. Liverpool don't normally do this, and they've signed a whole new front line and then you've got Mo Salah there as well.
"So they're all competing, they all want to do well, they're all fighting against each other to get in the team and I think Wirtz is the one who's probably not doing as well.
"He's got so much ability and for me, I don't see where he gets in the team.
"I think it was a lot of money, I would have went for Ekitike and Isak. I think Wirtz actually damages the balance of Liverpool, how they play.
"But he's a top player, I'm sure he will get better, of course he will. But he's had a slow start and there's no denying it."