"He is improving in the last month", Sarri told BT Sport.
While Eden Hazard has already predicted his team-mate will develop into one of the best in his position in England, the coach went one further. "He has been able to train after the problem with the back".
Giroud's hat-trick was Chelsea's second in the competition this season, after Ruben Loftus-Cheek in the 3-1 group stage win over BATE Borisov last October. He has scored nine goals in this competition.More news: Robert Lewandowski: Bayern Munich didn't take enough risks against Liverpool
"The pitch is not good and I think that it's really unsafe to play any football here".
"I don't know the reason, probably winter, because I played here two years ago and the pitch was wonderful (but it is now) very, very unsafe". Physically, technically and tactically he is improving.
Giroud's ideal hat-trick means he is the Europa League's top scorer this season with nine goals and Sarri was delighted with his display.More news: Sephora urged to drop partnership with Lori Loughlin's daughter Olivia Jade
"He played very well - better in the second half because we found the spaces and Callum in the spaces is really a very good player", Sarri added. As you know very well, Olivier is a very important player. First of all, because it means we are in the final and I am happy with that, ' said Sarri.
The Premier League side will now await Friday's draw to see who they will face in the quarterfinals, with a spot in the Champions League on offer for the victor of the second-tier European continental tournament.
Sarri was equally delighted with the performance of his side, who did not let their 3-0 lead coming into the match deter them from equalling their biggest European away win since the 1970s, scoring five on the night in Ukraine without reply.More news: 'Empire' Actor Jussie Smollett enters not guilty plea
"We scored after five minutes which was a very hard situation for the opponents". I think that we played a very, very good first half.