The last the time the Blue Jays acquired a third baseman from the A's, it worked out pretty well: Josh Donaldson came over in 2015 and won the American League MVP Award as Toronto reached the ALCS.
With the offseason additions of Chapman, Kevin Gausman and Yusei Kikuchi, plus full seasons from Jose Berrios and Alek Manoah to offset the losses of Robbie Ray and Marcus Semien, the Blue Jays may now enter as the favorites in an absolutely loaded AL East.
Whether they win the division may rest on which version of Chapman they get at the plate:
2018 to July 22, 2019: .279/.359/.525, 21.8% SO rate
July 23, 2019, to 2021: .194/.306/.431, 27.0% SO rate
Chapman brings his Gold Glove defense regardless and is under team control for two seasons, but in 2018 and 2019 he finished in the top 10 of the MVP voting after earning 7.6 and 7.7 WAR. In 2021, after hitting .210/.314/.403 with 202 strikeouts, his WAR fell to 3.5, so there is a four-win swing in potential outcomes in Chapman's value, which could be the difference between winning the AL East or not (hopefully not the difference between making the playoffs or not after the Jays fell one victory short in 2021).
My colleague Bradford Doolittle's projection system does, indeed, forecast the Blue Jays as the best in the East with 98 wins -- three more than the New York Yankees.