Valladolid spent most of the first half on the back-foot, but once Charles Dias had been sent off early in the second half the hosts took control and Guerra's goals gave them a hugely important 3-0 win.
The result sees Valladolid move to 16th place, two points clear of the bottom three and level on points with their opponents, who are one place higher on goal difference.
Celta Vigo created plenty of the early running, but Dias curled an effort over before a further effort by Augusto Fernandez was likewise off-target.
Guerra had a good sight of goal for Valladolid in the 19th minute but Gustavo Cabral made the block.
Fernandez then drilled a powerful effort wide before Diego Marino used his feet to keep out a shot from Nolito who should have done better from a good position.
Dias then enjoyed the best chance of the game with a powerful header, but Marino did well to turn it around the post at the expense of a corner.
Valladolid were struggling to carry a significant threat themselves, with Patrick Ebert shooting weakly when well placed in the 34th minute.
Instead, Celta continued to press but another curling effort from Dias was off-target before the break.
Things changed in favour of the hosts in the 56th minute when Dias fouled the goalkeeper and was dismissed for a second bookable offence - having also brought down Alvaro Rubio midway through the first half.
Without the man who had carried so much of their threat in the first half, Celta found themselves on the backfoot and then behind seconds after the hour mark.
Ebert turned provider this time, playing the ball in from the right for Guerra to slot home.
They doubled their lead seven minutes later when Carlos Pena crossed from the left for Guerra to head home.
Rafinha tried to get the visitors back in the game in the 75th minute but his curling effort sailed over the crossbar.
Jesus Rueda went close to a third for the hosts soon after as he met Ebert's corner, but Yoel Rodriguez held on.
Rafinha had a chance to get Celta back in the game with five minutes to play but Rueba blocked the shot and moments later, Guerra's third of the night killed them off completely.
Oscar Garcia was the provider this time, crossing from the right to leave Guerra to finish from close-range.