Fabio Carvalho, Curtis Jones and Kostas Tsimikas struck the goals for Liverpool in their preseason friendly win.
Slot told liverpoolfc.com: "We are very happy with the result, we scored some really nice goals. But I don't think we had enough control over the game because United were threatening us more than a few times. It's a 3-0 win but the score could have been different tonight as well.
"Of course the results, that's where everybody is looking at. But players stayed fit and they are able to perform in the way we want, so high-intensity game. We saw some great goals during this tour, some great build-up situations and I see them working really, really hard not to concede.
"Like I said, today I think we gave away too many chances, United deserved more than a 3-0 loss. But it's also our quality that we do score our own chances."
Slot added: "We need to work hard in the upcoming two weeks to be ready for Ipswich because there are a lot of positives to take from today but also, like I just said, we gave away too many chances."