The 40-year-old forward declared his intention to return to the NHL for a 22nd season last month after spending some time mulling retirement. He had 51 points (16 goals, 35 assists) and averaged 15:33 in ice time in 73 games last season.
"My understanding is that Joe Thornton is going to play," DeBoer said. "I saw him two days after our season ended in the gym working out. I think he's on record as saying he's going to play and still has gas left in the tank.
"For me, I'm moving forward as if he's going to be in our lineup in September."
Video: SJS@STL, Gm3: Thornton buries rebound on backhand
Thornton spent each of the past two offseasons working his way back from knee injuries. He tore the ACL and MCL in his left knee in 2017 and tore the MCL in his right knee in 2018.
Thornton ranks eighth in NHL history with 1,065 assists and 14th with 1,478 points in 1,566 games. He had had 10 points (four goals, six assists) in 19 Stanley Cup Playoff games and has 133 points (31 goals, 102 assists) in 179 career postseason games.
DeBoer was less committal about the chances of veteran forward Patrick Marleau, also an unrestricted free agent, rejoining the Sharks.
"I think he has something left in the tank but that's a situation that is in Doug's hands right now," DeBoer said, referring to Sharks general manager Doug Wilson.
Thornton and Marleau were the faces of the Sharks from Nov. 30, 2005, when Thornton was acquired in a trade with the Boston Bruins, until July 2, 2016, when Marleau signed a three-year contract with the Toronto Maple Leafs. Marleau was traded to the Carolina Hurricanes along with a conditional first-round pick and a seventh-round pick in the 2020 NHL Draft by the Maple Leafs for a sixth-round pick in the 2020 Draft on June 22 and five days later had the final season of his contract bought out by the Hurricanes.
One player who definitely won't be back is former captain Joe Pavelski, who signed a three-year contract with the Dallas Stars on July 1. Replacing Pavelski's 64 points (38 goals, 26 assists) was one of the topics DeBoer addressed in a wide-ranging discussion.
How difficult was it for you and general manager Doug Wilson to see Pavelski, the Sharks captain the past four seasons and one of the backbones of the team, leave as a free agent?
"It was a business decision. That's the tough part of Doug's job. He has to make those decisions. In a coach's mind you don't have to factor in money and (NHL salary) cap and people who need to be signed a year or two down the road and where the cap's going. We think with our hearts. In our mind a guy like that is irreplaceable. But the fact is, Doug did what he has to do. The big thing is, Pav (Pavelski) understood and Doug understood. How much of a loss it'll be to our group will be determined, but it's a huge void to fill."
Video: Pavelski discusses new beginning with Stars
How do you expect that void to be filled?
"We've got to continue to hope that guys like Timo Meier can build on the season he had last year. Kevin Labanc too. I think Doug has done a great job of setting us up with young players in the pipeline we feel can create some offense. Look, it's hard to replace Pav's 38 goals. We scored a lot of goals last year and if we score a little bit less I don't think it will kill us. At least I hope it doesn't. The goals are one thing, but it's the leadership, the presence, the message that he would convey in the dressing room when times were tough. Those are the things that are harder to replace than his goals."
While Pavelski is now with the Stars, the Sharks were aggressive in signing defenseman Erik Karlsson to an eight-year contract. How much confidence does it give you as a coach to know that the core of your defense of Karlsson, Brent Burns and Marc-Edouard Vlasic all are locked up long term?
"When we had the opportunity to acquire Erik Karlsson last summer there was no hesitation in anyone's opinion to go forward on the possibility of doing that. Those are generational-type players and they rarely become available, if at all. It was a no-brainer to trade for him and it was a no-brainer to sign him. He's going to be a huge part of what we're doing going forward. You take out Pavelski but you add Karlsson and some young guys. … The game might change in how we create and how we do things but I think he's going to have a big impact. The two months he was healthy he controlled a lot of the games we played. We just need to get him healthy so he can have a full healthy year to get into rhythm."
Video: Erik Karlsson commits long term to Sharks
You are entering your 25th season as a head coach (13 in the Ontario Hockey League, 11 and counting in the NHL). How important is it for you to pass on some of that knowledge to the young coaches attending the 2019 Hockey Coaches Conference in Toronto, which starts Thursday and where you will be a keynote speaker?
"I can only speak to my own personal coaching foundation and it was laid out in the coaching clinics I attended in the summers and the Hockey Canada experience. That was my chance every summer to listen to coaches at higher levels, pick their brains and make myself a better coach. I'd take things from them that I could implement and be excited about for the following season. That was part of mine and (Sharks assistant coach) Steve Spott's and (Winnipeg Jets coach) Paul Maurice's background. That was part of our summer offseason every year, was attending those things. It always amazed me at the end of the weekend how many things I took from coaches at all levels. It wasn't just the NHL coaches; you'd listen to coaches from a junior tier or from college and you'd take away some things they said that you could use. That was where my foundation as a coach was laid. It's tough to give up a weekend in the summer but it was critical for me to be part of this. The other thing is, the networking you can do as a coach here is important. It's a great way to become familiar with young coaches. You never know down the road when you need to hire assistants and know some of the younger guys you met at these things."
What is the most important message you'd like to pass on to the coaches in attendance at the event, which runs through Saturday?
"I think the biggest thing is a quote I once heard that becomes more and more important every year for me. It is, 'The players don't care how much you know until they know you care.' Sometimes that gets lost in today's landscape of video and X's and O's and analytics and how we break the game down. When I think of that quote I think of coaches like Pat Quinn and Brian Kilrea. As we move towards the more technical parts of today's game I think it's important we don't lose that aspect of it."