Robertson scored one and created another in the victory.
Slot said of the veteran left-back afterwards: "If you talk about a goal and an assist, the first player you think about is probably not a full-back.
"So, if he can then add that to a game where we until 91 minutes again did not concede a chance - and he's, of course, part of defending, so that's the first job of a full-back - then it's really positive. But the first goal he scored is not a big chance; it's a 20-yard shot, which we had a lot of three days ago - and now this one goes in after Cody (Gakpo) and Mo did a good job in the lead-up to that goal.
"The second goal follows straight away: again, Cody having a good role in that attack, dropping low as a nine, switching it to Rio (Ngumoha), a great overlap by Robbo. Mo, he's hardly ever offside - that is probably one of the most special things.
"He has a lot of special things, but that's definitely one of them, so when the linesman raised his flag, I was straight away questioning him - and that seemed to be fair because he wasn't offside. Then we are 2-0 and then it opens up the game much more than three days ago. For the rest, I think, it was an identical game."
Robertson is off contract at the end of the season and Slot also said: "He loves the club and he loves to play in front of these fans. These fans were again amazing, by the way, three days after they were so disappointed. Like us, they had to go again on a Friday evening over here.
"I don't know if they were all the same, but there were definitely more because in the FA Cup you have even more away fans. Robbo has in the one-and-a-half years that I'm here, he has given his everything for the club. I think the fans will tell me that it's not only one-and-a-half years he did this. He did this for all the years that he's been here.
"Every time I need him, or the club needs him, he tries to give his best. That's again what he did tonight, not only scoring and having an assist, but I think he defended well with calm and a lot of composure on the ball. That's crucial because if you play against the team where the full-backs are constantly open you need in those positions (a) player who is making the right decisions, that's what Dom and Robbo definitely did tonight."