Allen returned from a 12-week foot injury through the reserves last week, but his form line was clouded by the torrential rain that reduced the contest to a soggy slog.
Allen kicked 12 goals in the first seven games of the season before his setback, as he looked to put two injury-hit campaigns in Perth behind him.
The two-time defending champions host Port Adelaide on Sunday, chasing a seventh-straight win.
Eric Hipwood has recently returned from a long-term knee injury alongside Logan Morris, while rucks Darcy Fort and Sam Draper rotate through the forwards acting as a third tall.
The Lions (12-6) are fourth with five games remaining, but Fagan said there was no urgency to slot Allen in "and make a change for change's sake".
"You only need a week or two, you can tell," he said of assessing a player like Allen.
"We're not under any time pressure at the moment.
"It's really hard to change a team that's winning ... it's a tough decision, not sure which way we'll go."
Fagan said he'd been encouraged by Hipwood's efforts in his return and that adding Allen into the mix as a third recognised tall wouldn't change the dynamic.
The coach said Dayne Zorko, who appeared hobbled against West Coast, had passed fitness tests and was expected to play against Port.
