"initially presumed" is a correct and usable phrase in written English.
You can use it to describe when something was thought to be true at first, before being proven wrong or replaced by a different assumption. For example: "The detective initially presumed the crime was the work of an organized group, but it was later determined to be committed by a single person."
Consider "Gravity," a story about a divorced father whose teenage daughter is missing, initially presumed kidnapped.
It was initially presumed that the Bucs would cut the troubled Talib.
But its successor suggests Oliveri's sacking may have damaged Queens of the Stone Age more deeply than was initially presumed.
Next came the sunny two-movement D major Trio (1799), whose gracious charm and elegant Classicism make clear why it was initially presumed to be by Mozart.
The calls and notes, investigators said, will be crucial to determining potential motives and gleaning details on the relationship between Mr. Malvo and Mr. Muhammad, who was initially presumed to have taken the lead in all aspects of the attacks.
The men were initially presumed to be onlookers when a photo of them emerged showing them looking after Rezgui as he walked away along the shoreline, gun in hand.
The development between Shane Walsh and Lori Grimes continues in "TS-19", which showcases a flashback featuring Shane being conflicted with the decision to leave an unconscious Rick Grimes in the hospital who Shane initially presumes to be dead.
Thus, we did not initially presume a positive or negative association.
Given the sub-study's original focus on the impact of the hygiene promotion activities, we pooled schools from both the HP&WT and Sanitation + HP&WT intervention arms for random selection, because both contained the hygiene promotion component, and we did not initially presume that the sanitation component would influence the results.
Initially, we presumed that the pink nature of the hypopyon was secondary to coincident red blood cells with the leukocyte infiltrate.
Initially, we presumed that chlorine originated from the saline in the blister package.
