Suddenly, Cleveland had a roster sporting three young All-Stars -- Mitchell, Darius Garland and Jarrett Allen. That trio, plus NBA Rookie of the Year runner-up and burgeoning star Evan Mobley, all were under contract for at least three more seasons.
It's the kind of nucleus NBA franchises dream of, without the immediate pressure to win that often follows.
But then the season started. And four months after racing out to an 8-1 start, the Cavaliers have found themselves in fourth place ahead of the regular season's stretch run -- and with an entirely new outlook.
"[Outsiders] looked at us as a young team, and you don't expect a lot from a young team," Allen told ESPN before Cleveland's 117-113 loss to the Boston Celtics Wednesday night. "You expect them to come in and try to figure themselves out.
"But I feel like we came in and made an impact on the league."
There's little argument that Cleveland has proven that. The Cavaliers are one of three teams -- along with Boston and the Philadelphia 76ers -- to sit in the top 10 in both offensive and defensive efficiency. Mitchell and Garland have formed one of the league's most dynamic backcourts. Mobley and Allen make up one of the league's most imposing pairs of rim protectors.
But none of that changes this group's collective lack of experience. The eight players who played in the first half Wednesday have played in a combined 11 playoff series in their careers.
"If you look at all the teams that are above us, they've all gone through [postseasons] multiple times and had multiple sets of failures, and then somewhere they've been able to figure it out," Cavaliers coach J.B. Bickerstaff said.
The three teams Bickerstaff referred to -- the Milwaukee Bucks, Celtics and 76ers -- are veteran-rich teams loaded with playoff experience. Naturally, expecting Cleveland to immediately be on their level, regardless of talent, is a stretch.
What's changed, though, is the landscape around the Cavaliers in the East, leaving the door ajar to join the conference's elite.
The Brooklyn Nets -- once expected to be among the top four teams -- are no longer championship-caliber after trading Kevin Durant and Kyrie Irving. The Miami Heat, Atlanta Hawks and Toronto Raptors, all nestled with Cleveland between 46.5 and 48.5 wins entering the season, according to Caesars Sportsbook -- are fighting for play-in positioning.
That leaves the fifth-place New York Knicks -- winners of seven straight games after routing the Nets Wednesday at Madison Square Garden -- as the biggest threat to Cleveland landing home-court advantage in the first round. (It also sets up the juicy possibility of Mitchell going head-to-head with the team that spent so much of the summer being discussed as his eventual landing spot.)
"I think we're capable of being a championship team," Mitchell said after Wednesday's loss. "I think the biggest thing for us, obviously it's not a secret, is that we lack experience. When people talk about we're 'Not in their top three,' or whatever it is, that's fine.
"I think the biggest thing for us is we believe we cannot just make the playoffs, but make a deep run, and this is all, like I said, a learning experience. ...
"It's about continuing to get better, so, when we get to the playoffs, whatever seed we are, we're ready."
Ultimately, Wednesday's game showed just how far Cleveland has to go.
Mitchell was outstanding, finishing with 44 points on 17-for-32 shooting -- his seventh 40-point game this season, the most by a Cavs player since LeBron James in 2009-10, according to ESPN Stats & Information. Garland added 29 points and nine assists.
The remaining Cavaliers shot a combined 1 for 11 from 3-point range. Mobley (12 points, 13 rebounds, 6-for-15 shooting) and Allen (2-for-5 for 5 points, 7 rebounds) were outclassed by Al Horford (23 points, 11 rebounds, 8-for-10 shooting overall and 6-for-8 from 3-point range) and Robert Williams III (11 points, 11 rebounds).
Jayson Tatum, for his part, exposed Cleveland's massive defensive hole at small forward, carving up the Cavaliers en route to 41 points on 13-for-21 shooting with 11 rebounds and eight assists in 36 minutes.
"They played some playoff basketball in that second half," Garland said of the Celtics.
"They punched us right in the mouth, and we waited too long to respond and punch back. Their intensity picked up, and that's where we have to grow.
"They've been here before, they know how to turn it up and bring the intensity, bring the fight."
For a team that hasn't made the playoffs without James in 25 years, the crucible of the upcoming postseason will both teach their young core what they need to improve, while showing Bickerstaff, general manager Koby Altman and Cleveland's front office where the roster needs to grow.
"I think we have the ability to beat anybody that's in front of us," Bickerstaff said. "But what we have to do is there's some things we haven't been through. There's some experiences we haven't had.
"My message to our guys is we'll go as far and as quickly as we learn from our mistakes. ... If we're able to flip switches quickly, we can go a long, long way."