But the Blues will welcome back fellow Grand Slam winners Cuthbert and Jenkins for Friday's domestic derby with the Ospreys at the Liberty Stadium and John sees their return as a big boost for the rest of his squad.
"We have got Gethin and Alex back and they will give the boys a lift," said John, who took over the reins in the capital following Phil Davies' resignation earlier this month and is expected to hand Jenkins the captaincy in Warburton's absence.
"Hopefully these boys will lead from the front and inspire the rest of the squad. We are missing Sam and Leigh but that's the nature of professional rugby. We are disappointed the pair of them are not going to be playing any part in the last six games but the squad have to pick themselves up.
"Gethin has been excellent when he has come back in and captained the side. He's a leader and having him back for us is a great help because we have a man of huge experience with the amount of games he has played in.
"His presence gives everyone such a lift. He leads by example and his workrate within a game is phenomenal. He is a leader by deed who does not say much but what he does say people listen to. He is not afraid to tell us what he thinks, which is great for us."
The Blues could also soon welcome back former Wales skipper Matthew Rees and lock forward Bradley Davies as the pair continue their recovery from testicular cancer and ankle ligament damage respectively.
Rees hasn't represented the region since October and Davies has been out of action for the past two months but the pair appear to be on the brink of a comeback according to John.
"Hopefully players like Matthew and Bradley won't be too far away and, if you ask them, they will be ready tomorrow," added John.
"We will keep in contact with the medical team but they are close. It is great to see Matthew back on the training field because players like that are role models. They set the example and increase intensity in training, which is great for us."