SYDNEY, AUSTRALIA - APRIL 22: Brian To'o of the Panthers is tackled during the round seven NRL match between the Penrith Panthers and the Newcastle Knights at BlueBet Stadium, on April 22, 2021 in Sydney, Australia. (Photo by Jason McCawley/Getty Images)

The Penrith Panthers backline for their Round 9 Western Sydney rivalry clash with the Parramatta Eels is in a state of uncertainty just days out from kick-off, with Brian To'o racing the clock to return, and both Stephen Crichton and Taylan May missing training on Monday.

The Panthers had just a five-day turnaround leading into last week's clash with the Gold Coast Titans, which served as their eighth straight victory to start the season.

The top of the table outfit have had to do most of their work without To'o up until now, who has been nursing an injury that he sustained during a Round 2 clash with the St George Illawarra Dragons.

It was reported last week that he would be on the verge of a return in the coming weeks, and The Sydney Morning Herald's Christian Nicolussi has reported that he trained on Monday at left wing.

The game's most dangerous winger when it comes to running metres, it has been speculated that To'o will likely take the spot of Charlie Staines, rather than Taylan May, who had originally replaced him at the onset of injury.

To'o will also have just weeks to lock up his Origin jumper when he returns, with clashes against the Eels and Storm in the next fortnight set to provide a real litmus test for the Panthers.

With May reportedly ill though, To'o could well be forced to replace the young winger this week if he is unable to to make it back to the park for the game. It's unclear at this stage if ill means May and Crichton have coronavirus, or another form of illness.

Should Crichton be missing for the game, either Christian Crichton, or one of two young gun development players in Thomas Jenkins or Sunia Turuva could be handed an opportunity in the centres.

Teams for the game with Parramatta will be confirmed at 4pm (AEST) on Tuesday, with kick-off set for 7:55pm (AEST) on Friday.