He retired from worldwide football with Spain a year ago after 102 appearances, a World Cup title and a European Championship crown.
The draw means second-placed Atletico Madrid could close the 12-point gap with Barcelona, who are on 74 points, if they beat Celta Vigo later on Saturday.
"It is very hard over the course of a season to always be at the highest level", said Valverde.
Messi's not the only Barca player who will have the weekend off ahead of the second leg of their quarterfinal against United at Camp Nou on Tuesday.
Barcelona secured a 1-0 win at Old Trafford but Messi left the pitch mid-through the first half of the game with gushing cuts on his eyelid and nose.
"I don't know if they did or if it was Messi who made a decision to be quiet", he told RT. "It's a good sign for Tuesday, as we must be ready and prepare for the potential of extra time too".
It took the Englishman just 30 minutes to wipe out Barca's star man on Wednesday night, crashing through the back of him to win the ball.
The Red Devils are faced with a daunting task to shock the tournament favourites as they look to overturn a 1-0 defeat suffered in the first leg.
You'd have to go back three years to find a similar case, but even then the numbers don't compare to this week's. "We want to show that we're superior".
Manchester United manager Ole Gunnar Solskjaer believes a Champions League comeback at Barcelona would be a greater achievement than the club's last-16 miracle against Paris Saint-Germain. No, we didn't create chances, didn't get on the end of big chances. As far as their opponent is concerned, Huesca isn't quite a tough competitor and with players like Coutinho, Suarez, and Dembele leading at the front, there are fewer chances of Barca losing against them.
"There is pride and confidence, but if we do not win we are stuffed".
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