The Reds won 3-2 thanks to a goal ten minutes into injury-time through teenager Rio Ngumoha. However, Slot felt the game was too stop-start on Monday night.
He said, "I'm not too sure it was a football match today. It was set-piece after set-piece, long- throw after long-throw. We didn't collapse and stood strong. We didn't play well enough on the ball. I don't think there was so much open play.
"It was a great goal for a 16-year-old. Rio can finish so well for his age. I did hear someway say afterwards in the dressing room he would have taken a first touch but he is so confident. For his age he is a really good finisher."
Slot also said: "Relief, because if you are 2-0 up against 10 men...they brought it back to 2-2 and maybe it was even deserved. Then they had more chances than we had. Their fans were so loud - wow! It was an amazing atmosphere. To end with a goal during the best attack we had...the feeling is relief. We found a way to win."
On conceding two goals, the Dutchman added: "If you want to be where we want to be at the end of the season, you have to start keeping clean sheets. We've conceded too many goals, but we have scored too."