BOSTON -- A day after his first outing in nearly a month, right-hander Edwin Jackson was designated for assignment by the Blue Jays on Tuesday. Righty Jacob Waguespack was recalled from Triple-A Buffalo and started against the Red Sox.

"Our bullpen's thin right now," Blue Jays manager Charlie Montoyo said. "So we needed pitching, and he wasn't going to be able to pitch for two days."

When a player's contract is designated for assignment -- often abbreviated "DFA" -- that player is immediately removed from his club's 40-man roster, and 25-man roster if he was on that as well. Within seven days of the transaction (it was previously 10 days), the player must either be traded, released or placed on irrevocable outright waivers.

The 35-year-old Jackson returned from the injured list on Sunday after being sidelined since June 18 because of a lower back strain. He made his first appearance on Monday, pitching three scoreless innings out of the bullpen. Jackson mixed in two strikeouts, three hits and one walk. It was his longest scoreless outing since Aug. 11, 2018.

"It was good to be back out there," the 17-year-veteran Jackson said after Monday's 10-8 loss. "Obviously the outcome wasn't what we wanted, but it feels good to be able to repay the bullpen a little bit and help pick them up in a situation where they needed me to pick up some innings."

After the Blue Jays allowed 10 runs in the first three frames, Jackson's scoreless fifth, sixth and seventh innings helped keep the game within reach for his teammates to make a four-run comeback attempt in the eighth.

"He was throwing harder, he was throwing more strikes and his slider was pretty sharp," Montoyo said. "He did a great job. He kept us in the game. We needed those innings, and he gave us three good innings."

The Blue Jays became Jackson's record-setting 14th big league team when they acquired him from the Athletics in May. Prior to going on the IL, Jackson made five starts for the Blue Jays, and he pitched out of the bullpen following an opener in his last two appearances. Jackson was 1-5 overall in the first half of the season.

"When I was away [rehabbing], I kind of just took a breath and relaxed and went out with the approach [of], 'It's a new start,'" Jackson said.

Jackson is looking to get back to the success he had in 2018, when he went 6-3 with a 3.33 ERA, the third-lowest ERA of his career, over 17 starts with the Athletics.

"[I] just want to get back and revert to having fun like I was last year," Jackson said. "Obviously, I know what I can do. I would be at home if I didn't think that I could do it. But [I want to] just go back to having fun, and whatever's going to happen, let it happen."

Jackson adopted a positive outlook for the second half of the season. He described his mindset as "free" and wants to let the game come to him rather than try to do too much on the mound.

"You're not thinking. I think that's the biggest thing when you're having fun as a baseball player," Jackson said. "You're going out, you're letting the game come to you, you're allowing it to happen. When people are going good, you don't know what you're doing, you're just kind of doing it.

"Once things start going bad, you kind of start searching. Or you feel everything and you start to get too mechanical and you start to get -- I like to call it -- too smart for your own good, because we all know what we want to do. Sometimes we overthink the process instead of taking a step back and let it happen."

When Montoyo spoke to Jackson about the Blue Jays' decision, he told Jackson he didn't have to come to the ballpark Tuesday to collect his belongings if he didn't want to. Jackson, though, was enthusiastic about going to Fenway and seeing his teammates. He arrived in the clubhouse before the game with a smile on his face to say his farewells.

"I'm very lucky I got to manage that guy," Montoyo said. "What a professional."

Waguespack made his third Major League start on Tuesday against the Red Sox, the opponent he faced the last time he pitched for the Blue Jays on July 3. Montoyo said the decision to call up Waguespack was not tied to his previous experience (a win) against the defending World Series champions.

"He was the one that was ready to pitch today in the rotation down in the Minor Leagues," Montoyo said. "It had nothing to do with Boston or anything. It was the right turn."

The rookie is 1-0 with a 5.00 ERA in two big league starts.