Both players attacked a loose ball on the ground during Sunday's clash at Marvel Stadium and Dockers star Serong made high contact with Croft.
The inexperienced Bulldogs forward was left sore from the incident but got up quickly to take his free kick and played out the match.
Still, the incident caused enough concern for it to be a talking point in the Dockers' dressing room after their 17.10 (112) to 14.13 (97) win, which locked in a finals berth.
"(Serong) mentioned it to me not long ago," Longmuir said post-match.
"I haven't seen it, didn't notice it. From what I understand, Croft got up and took the kick.
"There can't have been much in it. (Serong) said, 'Do you think there's anything in it?'
"I said, 'What?' He said, 'That bump on Croft'. That's the exact conversation."
A suspension would rule Serong out of Fremantle's first final in three years, after the gun midfielder was an important contributor in the crucial win over the Bulldogs.
Versatile teammate Corey Wagner faces a stint on the sidelines with a torn pectoral muscle, but the Dockers expect small forward Sam Switkowski will be fit to play in September despite a knock to his knee.
Longmuir said Wagner would likely face "a few weeks" out of action.
"(He is) disappointed, like all players are. He's such a great team man though, so he didn't let it take any shine off," Longmuir said.
"That's what we love about him - he plays multiple roles, puts the team first. It's disappointing for him."
Longmuir said he was "real confident" Nat Fyfe would be ready to play a larger role in September if called upon.
The two-time Brownlow medallist, who will retire at the end of the season, has been the Dockers' substitute in three appearances this month on return from injury.
"He's done a power of work behind the scenes," Longmuir said.
