The duo met up recently at the Texans' practice facility. They posed for photos with each other, with Aguero holding a football and swimming in Watt's No. 99 jersey and Watt wearing Aguero's bright blue City kit.
Watt was nearly busting out of the shirt, while making a soccer ball look like a golf ball in his huge hands.
"Perfect fit," the Texans wrote on a Twitter caption for the photo.
Fans had a lot of fun with the photo, saying Watt looked like a boyfriend who forced himself into wearing his girlfriend's shirts. Another Twitter user compared Watt to Mr. Incredible trying to put his suit on.
Watt, 28, is listed at about 6-foot-6, 290 pounds. Aguero, 29, is about 5-foot-6, 152 pounds.
Aguero is one of the world's top-rated strikers.
Manchester City is facing off against Manchester United on Thursday at NRG Stadium.
"Good day with my friend @aguerosergiokun," Watt wrote in a Twitter caption.
He later explained his soccer fandom, saying he cheers for Chelsea.
"I've known Sergio for a few years now, so today was great," Watt tweeted. "I'm a huge Premier League fan in general but Chelsea is always my team."
The four-time All-Pro recently released a line of menswear, which fits a little bit better than his new soccer jersey.
The JJ Watt Collection includes six button-downs and a few polo shirts. They are sold exclusively at Mizzen and Main. Each shirt is named after a street that was important in Watt's life.