Messi took a blow in the first half of Barca's 1-0 opening-leg quarter-final success away to United at Old Trafford.
The Barca captain was left bruised and bloodied after United defender Chris Smalling attempted to win the ball ahead of the five-time Ballon d'Or winner.
Asked about Messi after the game, Valverde told reporters: "Messi wasn't so comfortable after the blow.
"We have to examine him tomorrow to see how he is. He was strong, but he has quite a significant bruise."
The between the pic.twitter.com/sOwjVMq2oV— FC Barcelona (@FCBarcelona) April 10, 2019
Barca were far from their best but still managed to grab an away goal, United defender Luke Shaw deflecting a Luis Suarez header into his own net.
The 12th-minute opener gave Barca their first Champions League victory away to United following two draws and two defeats previously.
"When you see the atmosphere here and the team they have and it being the Champions League, it's hard to win here," Valverde said.
"It was a difficult game and complicated at times, but we had the clearest chances."
With Shaw unfortunately awarded the goal, Suarez's Champions League drought away from home continued.
"The value of what he does doesn't depend on whether or not he scores, but what he generates," he added.