However, a game against Hallfredsson's Frosinone, who have taken just one point from a possible 12 this season, offers a chance for Ronaldo to boost his numbers.
Hallfredsson, though, has spent 11 years in Serie A and believes that both Ronaldo and Messi would find it difficult to hit the net with their superhuman regularity in Italy, given the defensive nature of the game.
“Since he has been in Italy, he has had a big impact just on the image of Serie A alone,” Hallfredsson told Goal. “Serie A is more valuable with him in it. I think for many years, Serie A has been underrated.
“In Italy, it is difficult to score. If Messi or Ronaldo come to Italy, it is almost impossible to score 50 goals like they do in Spain because it is more defensive. I think now though more people will watch Serie A and see how strong the league is.
“I think his arrival is so positive for Serie A. I think it is going to be sold out [for Juventus]. There are going to be so many people and it will be crowded outside the stadium. It gives a lot to a city like that to see a big player come here.
“He is maybe one of the best players of all time and it will give a lot to the people. We look forward to it."
The Iceland international came up against Ronaldo at Euro 2016, in a game that ended 1-1, and he accepts that there is simply "no tactic" that can stop him.
"I have faced him in the Euros in France, in the first game, so it will be the same as that," he added.
“There are no tactics to stop him, we just hope he has a bad day, but he has so few bad days. We will just try our best.”
Hallfredsson is battling to keep Frosinone in the top-flight, but says he was too old to remain at Udinese, after spending over two years with the club.
“To be honest, after two and a half years in Udinese, I needed to move,” He added. “Udinese wants to play with a lot of young players. The director of the club came to me and said if I want I could change teams because I would not get the playing time I would want.
"They said that to me and Danilo, the captain and we are the same age. Why Frosinone? Well, they were just so interested, you know. They have the ambition to try and be in Serie A. I have been there with Verona. I have had some difficult seasons and I know what it is like to fight for your life.
“I like the project. The president is a really good person, when I met the director and president I thought they were good with their ambitions. They want to do things the right way and I came here to be a key player.
“I came here to give my best to stay in Serie A. For my own ambitions, I want to play all the games and to get the team to stay in Serie A. That was the project and I thought, why not?”
Hallfredsson has carved out a career in Serie A, after leaving Tottenham as a youngster.
He took the risk of joining Hellas Verona in Serie C, but helped the team return to Serie A and survive relegation in their first season after a series of promotions.
His form earned him regular call ups for Iceland and he has played at the World Cup and the European Championships, with mixed success.
In Russia in the summer, Iceland failed to get out of their group, while they reached the quarter-finals of the Euros in 2016.
He admits that he is a little bit disappointed about the country’s showing in the World Cup, but he believes that there’s more to come from Iceland.
"I think it was a little bit disappointing to be honest because we had a really good game against Argentina. We got a great point, but in the second game against Nigeria, we weren’t on our day to be honest.
“Maybe if we got a draw in that game it would be different. Then against Croatia, we had so many chances to score. They had already won the group, which made it easier, maybe. We had created so many chances in that game.
“We were fighting at 1-1, then they got this 2-1 goal which just killed us. Our aim was to go through the group and be second in the group. It was a little bit of a failure but we need to stay proud of our achievement.
“I think a small country of just 300,000 people like Iceland to go to a World Cup is just incredible. When I was younger, I didn’t even imagine I would go to a World Cup. I played in the first and the third games. I remember them both like yesterday.
“The tension was so high and the atmosphere was unbelievable. Even talking about it, gives me the feeling in my stomach. Just these two games alone live with me forever. I enjoy going to the national team, I have a lot of friends.
“I am still playing at the highest level and we have a new coach. If he picks me, which I think he will, I will continue. As long as I keep enjoying it this much, I will go to. Why would you stop? After a few years you could regret quitting to early. I will enjoy it while I can.
“I think that we can still do it. We showed that we can do it. We have the right mentality to keep going. There will be more players coming in. With respect to how few we are, we have produced a lot of players.
"Now, I think we will aim to go to the Euros and I think we can do it. We have almost the same team. There are not too many players who are too old to play. I even think we can improve with this team. Our aim is to go to this tournament.”