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I think it would be an exaggeration to call Lazio a top club. They are close but no, there is the big four and there are all the rest, even if one of the big four collapses (like Milan this season or Napoli before) it does not change the status.
By the way, Lazio have games with Atalanta and Roma ahead (with a break for Bodo-Glimt in the LE), these could be critical games that will determine their place (including participation in European cups).
Yes, the points difference between Roma, Lazio, and Fiorentina is very close, so if only one of the three clubs fails and loses, then it could change their position. So far, Roma's match results are quite good, up to 5 consecutive wins. While Lazio and Fiorentina are quite bad in the last few matches. And yes, some upcoming matches are difficult because both Lazio, Roma and Fiorentina will face clubs that are quite strong and troublesome too.