Saints centre-back Maya Yoshida's tackle from brought the Brazilian to the ground and ultimately forced Maurizio Sarri into an early substitution, but the extent of the injury is not yet known.
Blues fans will be praying for good news, however, as the London side can ill afford another attacker out for a lengthy period of time.
Pedro, Callum Hudson-Odoi and Olivier Giroud all missed the matchday squad for Wednesday's match, the latter having suffered an ankle injury against Crystal Palace while the former pair dealing with hamstring issues.
In addition, Danny Drinkwater has been struggling with a back problem, while Loftus-Cheek, who replaced Willian, has been out a back complaint of his own and had no time to warm up before replacing his Brazilian team-mate.
Cesc Fabregas shrugged off a neck problem to make the bench.
Chelsea have already made their first acquisition of the January transfer window, securing the purchase of Christian Pulisic from Borussia Dortmund , however the young American winger will spend the rest of the season on loan with the Bundesliga leaders.
More to follow...