After missing Chelsea’s last Premier League game against Southampton due to a thigh injury picked up while on international duty with Senegal, Mendy was back in the post against Sevilla in the UEFA Champions League on Tuesday night.
The 28-year-old put in an assured display on his return as Chelsea kept a clean sheet against the Europa League holders.
This was Mendy’s third game in a Chelsea shirt since completing a £22m move from Rennes last month as he kept his second clean sheet.
Lampard was impressed with Mendy’s performance and intends to start him ahead of Kepa Arrizabalaga and Willy Caballero.
Asked if Mendy is already Chelsea's number-one 'keeper, Lampard said: "At the moment yes; he's shown his quality. The way he's played, he's played very well, that's two clean sheets for him already.
"As it stands he is [the number one] but that's always up for grabs. But the way he's playing I'm very happy. I was pleased with the professional element to the win.
"Seville are a top team in European football, no doubt. The concentration and focus to get the result was big from the team.
"We made a lot of good decisions tonight, so it's a really nice place for us to build from. I think it's a positive result. The clean sheet was a big deal for us."
On Tuesday, Chelsea also included retired goalkeeper Petr Cech in their 25-man Premier League squad.
