Manchester United have been given the green light to target the Royal Antwerp keeper, a player on their radar since the window opened.
The expectation is that Lammens would arrive to compete with Onana for the No.1 shirt rather than replace him.
But if Altay Bayindir starts at Craven Cottage, sources admit it would be a clear signal the hierarchy are prepared to push Onana towards an exit.
That would be a huge moment at the end of the window and now Onana waits to discover whether he is simply fighting for his place in the team - or fighting to stay at the club.
Reinforcements are still needed across the squad, especially in midfield, but the rebuild has stalled. What began with Matheus Cunha and peaked with Benjamin Sesko has lost momentum just as the focus was meant to shift to strengthening the middle of the pitch.
Carlos Baleba was identified as the anchor Ruben Amorim wanted to build around, and United stretched their budget to try and prise him from Brighton. But their refusal to sell has forced the search elsewhere - and so far, every path seems blocked. Adam Wharton is untouchable at Crystal Palace, Morten Hjulmand is only available at full release-clause price, and whispers of Angelo Stiller are met with scepticism.
The bigger frustration lies in who has not left. Antony, Jadon Sancho, Rasmus Hojlund and Alejandro Garnacho were all made available for transfer, yet none have moved on. That logjam has frozen reinvestment and left the squad effectively stuck in neutral.
Amorim would love to work with Hjulmand again after their time at Sporting, but for now his hands are tied. Recruitment chiefs have been told to sit tight, and unless a late market opening appears, the midfield could look exactly the same when the window shuts.
The arrival of Lammens would at least provide some relief in goal, though it is also not the quite kind of deal to excite fans who have already lost faith in both Onana and Bayindir.
Whether in goal or midfield, United’s indecision is threatening to define their season. And at Fulham on Sunday, the starting line-up may reveal just how far they are prepared to go.
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