Leicester needed to beat Cardiff to still have a chance of going through to the last four and the turning point came in the 39th minute with the sides locked at 6-6.
Cardiff hooker Gareth Williams was yellow-carded and Leicester scored 15 points in his absence including tries for Tom Varnell and Johne Murphy.
Leicester went on to score three more tries with Murphy and Dan Hipkiss securing a bonus point before a penalty try piled on the misery for Cardiff and Leicester ran out 42-20 winners.
Richard Cockerill was in charge for Leicester and will pick the team for next week's Heineken Cup opener at Leinster before handing over to former Argentina coach Loffreda.
Cockerill was pleased with the win over Cardiff, but knows there is plenty of room for improvement.
Leicester welcomed back 10 internationals and Cockerill said: "We knocked off the rust in the first half and found our stride in the second half.
"I told them to be patient at half-time and the attitude was second to none. The score was a bit flattering.
"We defended well in the first half, but we have to look at why we had to defend for so long. We made fundamental errors and they cost us in big games.
"The new boy (Loffreda) is coming in and I'm relatively happy with what I've done."
Cardiff head coach Guy Young was unhappy with the referee's decision to award Tom Varndell's opening try for Leicester after consultation with the television replays.
But he had no complaints with the result that leaves his side needing to win at Bath and hope Leicester slip up against Sale to go through to the semi-finals.
He said: "The first try didn't look like a try to me. But they stepped up a gear in the first 15 minutes of the second half and were immense.
"We were the better team in the first half, but didn't take our opportunities when we got them and you have to do that when you come to places like Leicester."
Young has injury worries with Gareth Thomas (knee) among those who had to be replaced.