Simons began on a flank before being moved inside, though was ineffective before his second-half substitution.
"I think Xavi, he had some good actions," Frank said.
"Also carrying the ball forward and they had to foul him three or four times. But there was a lot of situations where he was not as sharp, I think that's fair to say. I think he would say the same himself.
"But I also think it's natural. You're coming into a new team, a new league. To find that rhythm quick is difficult. And I also think that we said the other time, I think we're very happy with the Brighton performance in many ways.
"I think it's trying to get the right balance into the team.
"For me also to learn the players and find out how we can get the best players on the pitch at the right time.
"So that I'm of course looking into. I'm very aware that Xavi he can play as a 10. He also got in as a 10 against Brighton because that was the third game in a week.
"And I think that was positive. I think today when he got in as a 10, it didn't really work. And that was not only because of him, by the way. I think the team, the whole team was out of sync in the last 35 minutes or something like that.
"But of course, I try to look into what is the best midfield."