Tomori, 19, has been part of the Chelsea sides that have enjoyed remarkable success at youth level over the last three years, winning the FA Youth Cup and Uefa Youth League twice in recent seasons.
The full-back was also part of the England side that won the Under-20 World Cup this summer and was named on the first-team bench by Antonio Conte in Chelsea's opening three matches of the new Premier League campaign – although he is still waiting for his senior debut.
The deadline day arrival of Zappacosta from Torino means he will have to wait for that opportunity, but he will get the chance to cut his teeth in the Championship during Hull's bid to secure an immediate return to the top-flight.
He becomes the third Blues player to make a move to the KCOM Stadium, linking up with fellow academy graduate Ola Aina and former Reading defender Michael Hector.
Tomori's loan exit was always dependent on Chelsea bringing in another wing-back to provide cover and perhaps competition for first-choice option Victor Moses. Conte had earmarked England international Alex Oxlade-Chamberlain for that role with Arsenal accepting a £40m ($51.8m bid from their London rivals on Tuesday.
Oxlade-Chamberlain's desire to move to a club that will afford him chances in his preferred role in central midfield saw him reject the Premier League champions, however, with Liverpool swooping to sign the 24-year-old on transfer deadline day.
Chelsea then turned their attention to 25-year-old Zappacosta, who completed a reported £23m move to Stamford Bridge on an extremely busy night for the club which also saw Danny Drinkwater arrive from Leicester City and Ross Barkley's proposed move from Everton collapse at the 11th hour.
Amid efforts to strengthen the first-team, there was also a slew of loan departures that preceded Tomori's move to Hull with Chelsea confirming the permanent departures of Kyle Jameson and Cristian Cuevas to West Bromwich Albion and FC Twente respectively. Young midfielder Charlie Wakefield joined League Two Stevenage for the rest of the current campaign and Brazilian Nathan also completed a move to French club Amiens SC - a deal which includes the option of a permanent transfer.
Earlier on deadline day, Jordan Houghton joined Doncaster Rovers on loan until January with 18-year-old Malaki Mars returning to Barnet on a permanent deal.