The Bundesliga champion is rumoured to be among a number of clubs, including Paris Saint-Germain and Real Madrid, keeping a close eye on the Germany international.

Rudiger is into the final year of his Chelsea contract and will therefore be able to sign a pre-contract agreement with foreign clubs from January unless fresh terms are agreed.

According to reports that emerged on Friday (AEST), Bayern has already been in contact with Rudiger's agent about an end-of-season switch to the Allianz Arena.

But Nagelsmann was unwilling to discuss his side's supposed interest in Rudiger any further during Friday's (AEDT) news conference previewing the trip to Greuther Furth.

"I'm not talking about players from other clubs, particularly outside of the transfer window," he said. "But I will speak of him as a football fan: He has developed very well.

"Thomas Tuchel has coached him well. Before he was sidelined at Chelsea but now he is a crucial player. I can't saying anything else.

"Believe it or not, I haven't spoken to Brazzo [sporting director Hasan Salihamidzic] about it yet. Even if I had, I wouldn't give that away. 

"I have to see how long he still has to go on his contract. Then I will see if I have any money in my wallet."

Rudiger's importance to Chelsea since Tuchel took charge on 26 January is reflected in the statistics.

The 28 year-old has helped the Blues to 20 clean sheets in 32 matches in all competitions across that period – the most of any player in Europe's top five leagues.

Andreas Christensen and Cesar Azpilicueta are next on the list with 15 each, highlighting Chelsea's defensive solidity under Tuchel.

Rudiger has played seven times this season and has won 0.55 tackles per 90 minutes, compared to 0.76 for Niklas Sule, 0.88 for Lucas Hernandez and 1.64 for Dayot Upamecano among Bayern's regular centre-backs.

The German's 73.77 touches per 90 minutes and 56.86 successful passes is also fewer than each of Sule, Hernandez and Upamecano have managed, albeit in different competitions.

Sule and recent acquisition Upamecano have been Bayern's most regular pairing at centre-back this season and may get the nod for the trip to Trolli-Arena. 

Bayern enters the game on a seven-match winning run in all competitions, scoring 37 goals across that spell, having lost only one of its last 48 league games against promoted opposition.

Nagelsmann has Serge Gnabry and Jamal Musiala back in contention following recent lay-offs, but he will not take any risks over the fitness of the attacking pair.

"The good news is that Gnabry and Musiala have both been able to return to training, with Hernandez also taking part today," Nagelsmann said. "Serge and Jamal are both looking good and ready to play. But we have to wait and see what happens tomorrow.

"Both of their conditions could cause a reaction after training, so we have to wait and see how sensible it is to field them from the start. We also have a Champions League game coming up next week.

"Looking at today's training session alone, I'd say that both players can start tomorrow. But the probability they will both start is very low."