Castrogiovanni ended an eight-month injury absence from the Tigers first-team when he came on as a 59th-minute replacement for Julian White in Saturday's 20-3 Guinness Premiership.
The Italy international prop was given a rapturous reception by the Welford Road support as he entered the pitch.
Castrogiovanni said: It's been a long time, nearly eight months. I was really nervous before the game but as soon as I got on and heard the crowd it felt special.
When you hear something like that it makes you take your energy and feel really special. I really want to say thank you to everyone.
Castrogiovanni, who has been capped 54 times by Italy, has been out since last season with groin and shoulder problems.
The 27-year-old praised the form of his fellow props this season and admits he faces a tough battle to win his place back.
He said: I think Whitey, Coley and Marcos have all been outstanding. For me it won't be easy to get back into the team.
I need to keep working hard and training well because I think Whitey is playing better than ever, and it's the same with Marcos and Coley as well.
That's good for the team and it is good for me. It makes me feel I need to train harder, harder, harder to get back into the team.
The fitness will come. I need to keep training really hard to get some games in my legs and feel better.