Bonino has been using crutches and wearing a walking boot on his left foot. He was injured blocking defenseman P.K. Subban's shot in the first period of Game 2 at PPG Paints Arena, a 5-1 Pittsburgh win on Wednesday.
Bonino practiced Sunday but didn't participate in the Penguins' optional morning skate Monday and was seen leaving the arena using the crutches and wearing the boot at the same time several Penguins were on the ice for the morning skate.
The Penguins lead the Nashville Predators in the best-of-7 series, 2-1.
Bonino scored two goals in Game 1, a 5-3 win, and has seven points in 21 games in the 2017 Stanley Cup Playoffs.
"We utilize him in so many aspects of our game," Sullivan said of Bonino. "He's a real good player. He's just a real good solid two-way center iceman that we know makes our team more competitive when he's in our lineup."
Bonino, who plays on the Penguins top penalty kill unit and their second power play unit, was a full participant in practice Sunday. However, he said after practice that starting, stopping and pivoting were difficult. He said he felt better when he got moving at a higher speed.
Bonino said he was hopeful that he could play Monday, but insisted he would not campaign to do so if he didn't feel like he could help the team. He also left the arena using the boot and crutches.
"Obviously when you take [Bonino] out it changes things a lot," Penguins center Matt Cullen said. "He does a lot for us in every aspect of the game. He plays power play, penalty kill. A really intelligent player, competitor, does a lot of things right for us. So, yeah, if he can't play obviously it's a big hole to fill but it's one of those things that either way we've been doing it all season long. If we're called upon to fill his shoes we have to. Obviously they're big shoes to fill but we have guys that are capable."
It's unclear who would come out of the lineup if Bonino is able to return.
Left wing Carl Hagelin replaced Bonino for Game 3 and played 13:14, including 2:24 on the penalty kill. He had two shots on goal, four shot attempts and was credited with two blocked shots.
Carter Rowney took Bonino's spot as the third-line center in Game 3, moving to the middle from the right wing, where he had been playing. Rowney played 12:58 and was credited with a team-high five hits. He also had two blocked shots and won seven of 12 faceoffs.
Scott Wilson has played the fewest minutes of any Penguins forward in the past two games (21:29).
"I think it probably puts a little more pressure on everybody to step up a bit [if Bonino is out]," Cullen said. "For us it's all about our team game right now. We're working to just get our best game out there. That's the most important thing on our minds."
In addition, Cullen and right wing Patric Hornqvist each participated in the optional morning skate Monday. Neither practiced Sunday.
