The two teams have been discussing a deal for a while now, and with that in mind, the Blue Jays put in a claim when Johnson was recently placed on waivers.
The move gives the D-backs some much-needed depth in the middle infield. A season-ending injury to shortstop Stephen Drew last month has forced the club into playing Willie Bloomquist at short more than is optimal.
"One of the things we didn't like was to put that much activity on Willie Bloomquist at shortstop," said D-backs manager Kirk Gibson. "We were already playing him too much. [McDonald is a] very good defender at shortstop, he's a very spirited, fiery guy."
McDonald, 36, was batting .250 with two homers and 20 RBIs this year in a utility role. He owns a career .240 batting average in 13 seasons with Cleveland, Detroit (where Gibson was his hitting coach) and Toronto.
"My opportunity to play for the Diamondbacks for the next five to six weeks, plus the postseason, is a great opportunity, an opportunity I wake up every morning thinking about," McDonald said. "It's what every baseball player wants."
The D-backs are hoping that Hill is able to find the stroke that allowed him to hit .286 with 36 homers and 108 RBIs in 2009. So far this season, the 29-year-old has hit just .225.
Another plus for the D-backs is that Hill strikes out with less frequency than Johnson.
"I think the guy we moved and the guy we acquired are similar animals," Towers said, comparing Johnson and Hill. "If you looked at Kelly, it was kind of a peaks-and-valley-type guy, you just didn't know year to year what you were going to get. [Hill] is a guy that makes probably more contact, a guy that will probably give you a better [at-bat], and knowing that the power has been there before and at his age, the power can come back.
"You go into a ballpark like ours, most guys' numbers do improve once they go to the National League -- especially leaving the AL East. I think there's a chance his numbers can spike a little bit. We're not looking for this guy to carry this ballclub on his shoulders right now."
Though advanced defensive metrics give Johnson the edge this year, Towers said he feels the D-backs upgraded.
The combined salaries of the two players acquired roughly equal what Johnson would have earned in Arizona. Johnson and McDonald will be free agents this offseason, while Hill has a club option for $8 million.
It did not sound like the D-backs planned on re-signing Johnson next season and would have been hesitant to offer him arbitration in the fear that he would have accepted it after a down year in 2011.
"Going forward, the writing is kind of on the board by making this deal, that it was kind of doubtful that we would bring him back," Towers said.
With just over a week until the Aug. 31 deadline for setting playoff rosters, Towers did not rule out making more moves.
"I'll keep working the phones until the end," he said. "Any edge that we can get, if it means there's another move that we make in the next couple days to maybe make us a little better, we'll definitely take a hard look at it."
The D-backs played with 24 players Tuesday against the Nationals and will need to make a move to clear space on the 25-man roster Wednesday, when Hill and McDonald join the team.
To make room on the 40-man roster, the D-backs transferred right-hander Jason Marquis from the 15-day to the 60-day disabled list.