Hato, 19, has emerged as one of Europe's most promising young defenders after an impressive campaign in the Eredivisie, making 31 appearances.
BBC Sport understands that Chelsea are in direct communication with both Ajax and Hato's camp to explore the possibility of a summer transfer. Talks are understood to be in the early stages, with no agreement yet in place.
Meanwhile, Chelsea are discussing personal terms with Simons.
The Netherlands international, 22, is understood to be open to a move to Stamford Bridge, as the Blues look to add more creativity and attacking flair.
RB Leipzig are aware of Chelsea's approach, though no agreement has yet been reached.
Simons, who joined Leipzig permanently from Paris St-Germain in January following two loan stints lasting a season and a half in total, is also attracting interest from Bayern Munich and Arsenal.
Sources indicate Bayern have an interest in Simons but are prioritising a move for Liverpool winger Luis Diaz.
Arsenal, meanwhile, have been made aware of Simons' situation as they consider their summer options.
