Arrizabalaga starred in Chelsea's 2-0 Premier League win over Aston Villa on Sunday where he produced a series of superb saves and denied Danny Ings what would have been an equaliser to ensure Potter's impressive start at Chelsea continued.
The Spaniard was second-choice behind Mendy in the last two seasons but has reasserted his authority at Stamford Bridge after the Frenchman's injury last month, having kept four straight clean sheets.
"It's better to let football decide. Kepa's done really well, he's been supported fantastically by Edu," Potter told reporters on Tuesday when asked if he had a clear first-choice goalkeeper.
"Kepa's form is really pleasing for everybody. It's nice we have competition in that area and we need players to push each other as that is how they get better.
"I've been really impressed with his character and personality. He takes responsibility and it is nice when those type of people get reward for their efforts."
Potter said there were no new injury concerns before Wednesday's short league trip to west London neighbours Brentford and added he would make a late decision on the availability of defender Thiago Silva.
Chelsea have won their last five games in all competitions - the last four without conceding - and remain unbeaten under Potter since he took charge in September.
"It's been less than six weeks, we've got a long way to go. It feels like six months sometimes," Potter joked.
"It's gone fantastic in terms of results but more than that, the feeling in the team is really positive, the way the group responded."
Chelsea are fourth in the league with 19 points from nine games, while Thomas Frank's Brentford are ninth with 13 points from 10 games.
"Thomas Frank has done a fantastic job with limited resources but they make the most of what have," Potter said. "The way they play fits the players they have and they are very organised.
"Ivan Toney as a centre forward is as dangerous as anybody in the league... They attack with clarity and defend with aggression and are really tough to play against."
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