Tomori last month secured a £25million move to AC Milan, for whom he featured on loan in the second half of last season.
The 23-year-old played 17 times in Serie A after joining on a temporary deal in January and helped Milan finish runners-up in the Italian top-flight.
Meanwhile, fellow centre-back Guehi is set to join Crystal Palace in a deal that will see the Eagles spend slightly north of £15m.
Guehi has spent the last 18 months on loan at Swansea City and was an integral member of the Swans team who reached the Championship play-off final in May.
The 21-year-old was also part of the England team who won the U17s World Cup in 2017 and scored one of the goals in the 5-3 victory over Spain in the final.
However, he played just twice for Chelsea’s senior side and is now set to depart for Palace, where Arsenal legend Patrick Vieira has been appointed as head coach.
According to The Athletic, the departure of the pair has caused some concern among members of Chelsea’s youth set-up.
Prospects Tino Livramento, Lewis Bate and Myles Peart-Harris are believed to be stalling on new contracts and have not been included in the first-team’s squad for their pre-season training camp in Scotland.
Defender Dynel Simeu, another member of the Blues’ under-23s side, is apparently close to joining an unnamed Premier League team.
Chelsea have frequently developed exciting young talent before moving them on and including a hefty sell-on percentage clause in any deal.
Former boss Frank Lampard handed several senior debuts to academy graduates but that business plan remained the same.Chelsea boss Thomas Tuchel has continued to used Chelsea’s young stars after Frank Lampard’s sacking