Roy Hodgson’s team lacked a cutting edge in Sunday’s uneventful 0-0 friendly draw with the Republic of Ireland at the Aviva Stadium.
The result gave England a clean sheet and extended their run of games without losing to 10, a sequence stretching back to the 2014 World Cup finals.
Yet their failure to prise open the Irish defence was a more pressing concern prior to the trip to the Stozice Arena.
Already six points clear at the top of Group E in the qualifying stages, England will all but claim a place at the Euro 2016 finals with a win in Ljubljana.
Manchester United defender Smalling knows victory is anything but assured, however, after the manner of the Irish stalemate.
Hodgson’s visitors were too flat in Dublin and an upturn will likely be required against the country sitting second in the preliminary pool at present.
Smalling, 25, said: “It was a strange game and a strange atmosphere. It was really end to end. It was a strange game but one we could have done a lot better in.
“We felt like we lost the game, it’s not one we are happy with. I think the pace slowed up the last five to 10 minutes.
“We could have pushed it a bit more. We did have a lot of the ball but we didn’t create it too much.
“I think it’s one where we need to improve. It wasn’t a good performance from us and there are a lot of things we need to improve on.
“Considering we have had seven to 10 days off, I think we will make sure we are ready come Sunday.”
Smalling’s Manchester United team-mate Phil Jones believes the meeting with Ireland still served a purpose, even if it was a poor spectacle.
With the Premier League ending two weeks earlier, he thinks the match allowed the squad to ease back into action before a far more significant game against Srecko Katanec’s men.
Jones said: “We had 10 days off and we needed a game like that. We knew we were going to be put under pressure from the first minute.
“They’ve done that and they made it difficult for us. We weren’t at our best but we definitely needed that game.
“It’s a massive game for us we know what we have to do. We will go away and evaluate this performance and work on the things in training we need to get better at.
“We know the job we have to do. We have a few days now to prepare well for that. We need to go and do the business over there.”