Tampa Bay said Gaborik, who played his last NHL game in March 2018, and Nilsson, who is out after a concussion, "will be placed on long-term injury reserve and will not play this season," taking their contracts off the Lightning cap.
Gaborik has one year left on his contract at $4.875 million against the cap, while Nilsson has one year left and $2.6 million against the cap.
Tampa Bay eased its cap problems by moving Coburn, who has one year left on his deal at a $1.7 million cap hit, and Paquette, a depth forward with one year and a $1.65 million cap hit.
The Lightning confirmed to ESPN that Coburn, 35, waived his no-trade clause for Ottawa.
Coburn had four points (one goal) and 16 penalty minutes in 40 games with the Lightning last season, appearing in three playoff games. He averaged 14:03 in ice time. Paquette, 27, recorded 18 points (seven goals, 11 assists) and 42 penalty minutes over 61 games with Tampa Bay, and three assists in 25 playoff games.
According to Cap Friendly, the Lightning will have $16.975 million in cap space opened up with three players on long-term injured reserve to start the season: Gaborik, Nilsson and star winger Nikita Kucherov ($9.5 million AAV), who is undergoing hip surgery and will miss the entire regular season.
The combination of salaries moving in the trade and LTIR designations will get the Lightning under the $81.5 million salary cap.
The Senators, who acquired Arizona center Derek Stepan on Saturday in another cap selloff move, will save about $2.225 million in 2020-21 salary because of this trade with the Lightning.