Police say the main suspect, 38, is a local man with a string of previous convictions for criminal and drug offences.
The second detainee, 41, also from Poznań, is likewise known to police from his criminal record.
Both are due before prosecutors on Wednesday to be formally charged.
The attack happened last Friday at a tram stop in the city's Piątkowo district.
The victim had been travelling on a tram when he witnessed a group of men hurling insults at a Ukrainian boy and stepped in to defend him.
The men then turned on him instead, spitting on him and punching him repeatedly in the face.
He was hospitalised and is reported to be suffering memory problems as a result.
Officers are appealing for the boy, along with any other witnesses, to come forward.
Attack on two Ukrainian girls in Legnica
Separately, police in the south-western city of Legnica are investigating an attack on two 13-year-old Ukrainian girls near a public swimming pool on Saturday.
Interior ministry spokeswoman Karolina Gałecka said the assault may have been motivated by the victims' nationality, adding that no one has the right to use violence against another person, particularly a child.
The report was filed by the mother of one of the girls, who said an unidentified woman attacked them in the evening, reportedly in front of other children playing nearby.
Investigators are treating it as a case of bodily harm and are examining whether nationality was a motive.
The incidents are the latest in a series of attacks on Ukrainians in Poland.
(ał)
Source: PAP, IAR