Namestnikov, who signed a one-year, $2.5 million contract with Tampa Bay on July 13, finished the preseason with six points (five goals, one assist).
Nikita Kucherov and Brayden Point each had a goal and an assist, Corey Perry had three assists, and Andrei Vasilevskiy made 27 saves for the Lightning, who won their final game of the preseason (1-4-0).
Rudolfs Balcers scored, and Spencer Knight made 26 saves for the Panthers, who finished the preseason 2-4-0.
Kucherov scored his first goal of the preseason, a power-play goal from the right face-off circle, at 3:51 of the first period to give Tampa Bay a 1-0 lead.
Balcers tied it 1-1 at 5:15 with a slap shot from the top of the right circle.
Namestnikov made it 2-1 with a one-timer from the bottom of the right circle at 12:15, then extended it to 3-1 by banging in a cross-crease pass from Perry on a 5-on-3 at 19:14.
He completed the hat trick at 7:30 of the second period off a pass from Ross Colton.
Aleksi Heponiemi came out of the right corner and scored short-side to cut it to 4-2 at 8:45, but Point tapped in his first goal of the preseason at 4:56 of the third period for the 5-2 final.
