Krejci, who was an unrestricted free agent after 15 seasons with the Bruins, opted to play in his native Czech Republic for HC Olomouc this season instead of returning to Boston. That leaves a void at center, where the Bruins have been anchored by Patrice Bergeron and Krejci since 2007-08.
"I think the obvious choice is Charlie Coyle," Cassidy said. "He's the most familiar with our guys. I'm the most familiar with him. [It] allows the other guys to fall into place. ... Charlie and Taylor Hall, and [Craig] Smith were on that line last year. If Coyle can bring some of what [Krejci] did, it'll be a real good line."
Coyle did not have his best season in 2020-21, scoring 16 points (six goals, 10 assists) in 51 games after signing a six-year, $31.5 million contract ($5.25 million average annual value) Nov. 27, 2019. He scored 37 points (16 goals, 21 assists) in 70 games in 2019-20. Krejci scored 44 points (eight goals, 36 assists) in 51 games last season.
Cassidy said he is not interested in breaking up the top line of Brad Marchand, Bergeron and David Pastrnak. Given that, Coyle is the logical choice to take Krejci's spot, at least to start the season.
Bruins general manager Don Sweeney on Friday said there's a possibility Krejci could return.
"Ultimately with David making a decision, we were wondering whether or not he may return at some point and time, but that's an open-ended [situation]," Sweeney said. "... We'll see if somewhere down the road, Krejci opens the door back up. But again, that's his own family decision and there's no timeline. We did have a heads up in that and where we were in the process of trying to fill in from a depth perspective in free agency."
The Bruins also are trying to gauge where prospect Jack Studnicka would play in the lineup. The 22-year-old played 20 NHL games last season, scoring three points (one goal, two assists).
"How does he fit in? Some of that will depend on his growth," Cassidy said. "Didn't get to play a lot of hockey last year, unfortunately. Like a lot of young players, missed some time with COVID restrictions. He's also in the mix. He looks bigger, going to be really pushing for a spot. Have to include him in that mix as well."
Cassidy acknowledged that the Bruins are likely to miss Krejci but also said his absence would provide a chance for other players, whether it's Coyle, Erik Haula or Studnicka. This could be a push for those players -- extra minutes, extra responsibility -- to play with an elite player like Hall.
"[Bergeron] will still give us high-end minutes. We know that," Cassidy said. "David is the one that, he played a big role for us, sometimes quietly. Now, all of a sudden, when he's not here, we'll probably realize how good a player he was. We all knew it, but now we're going to live it firsthand without that 1-2 punch.
"But having said that, whoever goes in there -- let's say it's Haula -- these guys are excited to move up in the lineup. This is opportunity, right?"