After a stunning grand final triumph against Geelong in September, the Lions added ex-West Coast captain Allen and former Essendon ruck Sam Draper during the trade period.
Both signings have endured significant injury issues recently, with Draper rupturing his achilles in April when in career-best form.
But Fagan expects the 27-year-old to be available for Brisbane's opening-round match against the Western Bulldogs in March.
"The expectation is that they'll be right to play round one," Fagan told SEN when asked about Draper and Allen.
"In fact, they should be right to play at least one practice match, if not two.
"If there's no setbacks, that's what will happen."
Allen managed just 11 games in his final season for West Coast, opting to have knee surgery in June when he was already managing calf and achilles problems.
The key inclusions come in following the departures of premiership heroes Brandon Starcevich (West Coast), Callum Ah Chee (Adelaide) and Oscar McInerney (retirement).
Allen has arrived as a long-term replacement for Joe Daniher, who retired after the 2024 grand final win, while Draper could fit in alongside premiership ruckman Darcy Fort.
"They're both great characters," Fagan said of Allen and Draper. "They've both been in leadership roles at their clubs, which is great because we lost a little bit of leadership.
"They're training well at the moment, which is a bit of a surprise. I thought they might take a little bit longer.
"But they're back in, albeit a little bit modified. Doing really well and enjoying the new environment."
The Lions have the opportunity to complete the second premiership three-peat for the club this century, after Leigh Matthews famously guided Brisbane to flags in 2001-03.
Hawthorn (2013-15), who Fagan was involved with as football manager, are the only other AFL team since Melbourne in the 1950s to have won three-straight premierships.
"The group likes a challenge," Fagan said. "There's no better challenge than trying to win three in a row.
"Not many teams can do it. Hopefully they might be inspired by that, it's a pretty tough thing to do, but this group's got the ability to do it."
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