The 18-year-old was handed his first ever start as the Biancocelesti hosted Legia Warsaw in Europa League, and he largely impressed on his full debut.
Keita provided an assist for the winning goal and overall played a very good game at Stadio Olimpico.
According to Sky Sport Italia, Liverpool scouts were present in the stands as the Reds made a check on their long-term target.
The Premier League side have been keeping close tabs on the talented forward for a while now and they could decide to make their interest concrete very soon.
Keita, who joined Lazio from Barcelona two years ago, has a contract until June 2017 at Stadio Olimpico and is seen as a hot prospect for the future.
After losing the Capital One Cup tie to Manchester United on Wednesday, Liverpool face Sunderland away at the weekend, with Reds 3/5 to return to winning ways.