A story in the online Daily Mail says Manchester City, which manhandled perennial top Spanish club Barcelona on Wednesday, will see its deal with Nike end in 2018 and is looking for a more lucrative partnership.
The British newspaper speculates that Nike has committed large resources with other soccer clubs and may not want to go all in.
"In that case American giants Under Armour, with their UK base in Manchester, are the most likely suitors for City in the kit merry-go-round," the Daily Mail said.
An Under Armour spokeswoman said the company does not comment on speculation.
Seeking to expand its global presence, Under Armour has signed a number of international football clubs, including Tottenham Hotspur and Southampton FC.
The 2011 deal with Tottenham, currently sitting fifth in the Premier League, was its first big partnership with a European club, but it expires this year and multiple reports last spring suggested the London club was on the verge of signing with Nike.
The speculation around Manchester City and Under Armour prompted an influential kit artist who goes by Sebbulba to tweet out a vision for what Under Armour kits for Man City may look like.