Semedo lasted only 23 minutes during the game, before he was replaced by Sergio Busquets.
It had been a torrid week for the Blaugrana, who lost 3-1 at Levante before being held to a goalless draw by Slavia Prague, but Messi scored a penalty and two superb free-kicks as his side returned to the top of LaLiga.
"Because of this, the opponent can't commit fouls on the edge of their own box".
"Messi, apart from scoring goals, has a commitment to the game".
"Having him is a very big advantage".
The Argentina global delivered much more than just goals on a night when only a win would be acceptable for the Spanish champions, frequently dropping deep and driving an otherwise sleepy Barcelona side into attack.
"After losing, it's nice to win. Even more so in a team like ours", Valverde said in quotes reported by Marca. "The other day we had a painful defeat and we had to win today".
Messi completed his treble - matching the record of Cristiano Ronaldo of 34 in LaLiga - when he clipped in another impeccably executed deadball strike soon after the restart.
"It was really difficult for us in the first half, with their high press that made it hard for us to play out and we didn't have much rhythm in our play". With Griezmann and everyone else.
The defeat was Celta's fifth in a row and newly appointed head coach Oscar Garcia will have his work cut out as he tries to lift them out of the bottom three. There are lots of games.
The match is a charity friendly with proceeds to be used to support the victims of serious flooding that hit Murcia and eastern Spain back in September.