The Bruins (44-14-12, .714 points percentage) won the Presidents' Trophy with the best regular-season record in the NHL last season but lost to the eventual Stanley Cup champion Tampa Bay Lightning in five games in the Eastern Conference Second Round.
"[It] remains to be seen where we are as we move through camp and how he feels as a result of that as he introduces contact and such," Sweeney said of Marchand.
Marchand is a crucial part of the Bruins' top line, with Patrice Bergeron and Pastrnak, who had arthroscopic surgery on his right hip on Sept. 16; Pastrnak's recovery was estimated to be approximately five months, which would put his return in mid-February.
"David is certainly a longer timeline to return to play and be back on ice in a regular fashion," Sweeney said. "He won't initially integrate into the main practice group as he increases volume in his own return to play. It's unlikely that he starts the opening couple games, but we're going to monitor week-by-week and we'll continue to give you updates as they progress."
Pastrnak led the Bruins in scoring last season with 95 points (48 goals, 47 assists) in 78 games; he tied Washington Capitals forward Alex Ovechkin for the NHL lead in goals. Pastrnak scored 10 points (three goals, seven assists) in 10 postseason games.