The Italian winger has barely featured for the Premier League club since joining from Juventus on deadline day.
Chiesa has three appearances and just one start since a £12M move, having missed the last three match days.
"I think it is always so difficult with injuries," Slot said to reporters.
"Especially if it has just happened, to see where they are after a few days. I think after a few days you can say better where they are.
"With Federico it's a bit more simple, although it is difficult, it is a bit more simple to explain. He missed pre-season, I said this many times, and going to a league that is a higher intensity than Serie A, we just faced two Italian teams so I can say this now.
"That makes it difficult for him to make the step towards the intensity levels to where the rest of the team is at the moment. It has not so much to do with the Italian league or the Premier League but so much to do with him missing the entire pre-season.
"So it is difficult for every player, when the games are constantly there, to build up players to the levels we are at. It's difficult to say but it is a big disappointment that he is going in and out of the training sessions all the time.
"So I feel sorry for him but he signed a long-term contract, so we will see what he brings for us. At the moment, unfortunately for us he has been in the team selection one or two times and no more than that."