Weekes has played the first four games off the bench for the Sea Eagles in a utility role.
Able to play anywhere in the backline, Weekes has been explosive for Manly throughout his time off the bench, adding plenty of value in attack when he has had the opportunity to come onto the field.
With Josh Schuster to miss the game as he continues his battle to return from injury, head coach Anthony Seibold revealed to the media on Friday morning that Weekes will win out the race for the number six jersey.
“I think he has been really effective for us off the bench... he deserves an opportunity,” Seibold said.
Johns has struggled in his role at five-eighth though, and Weekes' call-up will be seen as a needed change for Sea Eagles' fans as they attempt to spin positivity back into their season after squandering an early lead in Mudgee last weekend.
Weekes was already named on the bench, so now it remains to be seen which of the reserves will slot into the bench.
That option comes with Karl Lawton - who can fill in at dummy half or the centres in a pinch - also named on the bench.
The other option for coach Seibold will be to name Johns as a bench player, or winger Raymond Tuaimalo-Vaega, who was also named in the squad on Tuesday.
Speaking on Schuster, Seibold said it was all about considering a long season and taking the cautious approach.
“We've tried to make sure we are looking after Josh and we want him to play against Penrith, but it is a long year,” head coach Anthony Seibold said.
“When he could feel it on Wednesday, we thought we should be smart about it.”
Teams will be cut to 19 at 5:30pm (AEST) on Friday for the clash between the Panthers and Sea Eagles, which kicks off at the foot of the mountains 24 hours later.