Pepe - who cost the north Londoners a whopping £72million - has only made one league start this season, with manager Mikel Arteta opting to hand him starting berths in the Carabao Cup and the Europa League.
"I think it was a game of two halves for Pepe," the former Gunners midfielder said during an appearance on talkSPORT. "I thought first half he was pretty poor.
"He picked up second half, but you can't do that (drift) when you are in big games you have to be effective for the 90 minutes. That's what Mikel Arteta is saying.
"You've got to play your part throughout the game. I think Saka does that. If you look at the way he plays he's always involved in the good parts of Arsenal's game going forwards and defensively.
"Pepe's just got to get that bit more consistency during the game because you can go missing.
"You can't have missing players because against the big sides you have to have everyone on that pitch doing their work and being involved in a good team performance, that's what he's probably trying to say there against Pepe.
"First half yesterday it was like his brother was playing. Second half he's turned up and got his goal. The price tag doesn't help, it was a lot of money. There's going to be more pressure when the fee is a little bit higher.
"I just want to see him do it five or six games in a row. He's got to have such a good game that Mikel Arteta can't leave him out."