Iago Aspas gave the Spanish side a deserved lead after 22 minutes, latching onto a pass from Svedberg to finish confidently from the edge of the box, and Sassuolo struggled to create chances of their own before the break.
Sassuolo hit back after the interval, drawing level on 57 minutes when Darryl Bakola turned home a slick team goal, released by Luca Lipani after Cristian Volpato's clever backheel had first opened up the space, and twice went close to taking the lead themselves – Andrea Pinamonti dragging a shot wide from a good position before Bakola crashed an effort against the crossbar.
The contest turned when Armand Laurienté was sent off for reacting to a heavy foul in the 76th minute, and Celta capitalised ruthlessly as substitute Gonzalez scored three times in the final ten minutes to complete his hat-trick and turn a tight contest into a 4-1 rout.
Head coach Alberto Aquilani took some encouragement from the response after a difficult opening period.
"These kinds of games are for training and this should give us pointers: in the first half we struggled, with long distances between the lines, but we stayed in the game. The second half was much better, our tempo and courage increased," he said.
"Bakola came on very well, he gave us the energy needed for a high press. Then after the sending-off, which can happen, I didn't like that the team switched off. There's an aspect of internal resilience there that stops you wanting to concede – that's less trainable than the tactical side, in football you have to know how to suffer."
On new arrival Kieron Bowie, who replaced Pinamonti: "He's a different player to Pinamonti, used to a different kind of football to ours, he needs to take on some concepts but he has potential."
Asked about the transfer market, Aquilani preferred not to elaborate: "I've already spoken about it, I'd rather not, otherwise we get into unpleasant topics. I trust the club, we'll see what we manage to do."
Pinamonti's own future looks increasingly likely to lie away from the club this month. With his contract set to run out in 2027 and no extension on the table, an August exit looks ever more probable; Sassuolo have already knocked back one offer from abroad in the past day alone, while Fiorentina and Lazio remain among the Italian clubs credibly linked with the forward.
Hull City are understood to be the club to have made a concrete approach, according to ModenaSportiva, with the Sassuolo hierarchy valuing the striker at around €15m, though the expectation is that a fee nearer €10m would be accepted to avoid losing him for nothing next summer.