The two riders, who "do not rank within the top 20 of the general classification", will neeed "complementary biological examination", the UCI said in a statement.
"While these riders are completely asymptomatic, security measures (isolation of the two riders) have been implemented to protect the other participants.
"A final decision on their participation will be taken by tomorrow (Tuesday) morning in a collegial manner by the concerned parties (COVID-19 event and team doctors, and the UCI medical director)."
The UCI did not clearly say whether those two riders had tested positive and the body was unable to be reached by Reuters for clarification.
Eight riders have so far pulled out of the race after being infected with the novel coronavirus and showing symptoms as part of the teams' international testing protocol.
Last year, if two members of a team tested positive for COVID the whole outfit would be sent packing, which is not the case in this edition.
Whether a rider who tests positive can stay in the race is decided following a meeting between race, UCI and team doctors, depending on the rider's viral load.
One rider is currently racing the Tour and is known to have had COVID during the race - that is Rafal Majka, a crucial teammate of reigning champion Tadej Pogacar.
There has been one further withdrawal so far on the third rest day - Jakob Fuglsang has abandoned the tour after a crash left him with a fractured rib over the weekend.
Jonas Vingegaard leads the race from Pogacar by two minutes and 22 seconds as they head to the Pyrenees during the last phase of the race.
