The Wales star had been left out in the cold for large parts of pre-season after a move to China collapsed, but he was handed a start at Balaidos on Sunday (AEST) and set up Karim Benzema for a 12th-minute opener.
Looking to capitalise on champion Barcelona's shock loss to Athletic Bilbao on Saturday (AEST), Madrid added a stunning second goal through Toni Kroos, but only after Brais Mendez had a strike ruled out and Modric was shown a straight red card.
Zinedine Zidane started without any of his new signings, although the quality of his side's third goal — rounded off by substitute Lucas Vazquez — will offer hope of a far brighter 2019-2020, despite conceding late on through Iker Losada.
Eden Hazard, Rodrygo Goes and Ferland Mendy all missed out through injury, while Eder Militao and Luka Jovic were named on the bench, so it was left to the old guard — not least Bale, Benzema, Kroos and Vazquez — to get the job done.
Madrid had lost back-to-back league matches at the end of last season, but it soon found its rhythm here as Bale turned his man inside out down the left and set up Benzema for a simple finish from four yards.
Celta felt there should have been a free-kick in the build-up and was further aggrieved in the final stages of the first half when Alvaro Odriozola was dispossessed by Denis Suarez and the ball fell to the feet of Iago Aspas, who was adjudged to be marginally offside when teeing up Mendez for a close-range finish.
Celta's hopes of claiming a point were boosted when Modric caught Suarez on the heel and was shown a red card after a VAR (video assistant referee) review.
Nestor Araujo forced Thibaut Courtois into a point-blank save but Madrid dug deep and scored a second thanks to Kroos' stunning strike from 35 yards, a minute after Benzema's header struck the post.
Vazquez rounded off a flowing move 10 minutes from time by drilling a good finish into the bottom-left corner, although Celta at least got a consolation through Losada's precise strike from a similar spot at the other end.
Madrid welcomes Real Valladolid to Santiago Bernabeu next Sunday (AEST), while Celta hosts Valencia on the same day.