For Shrewsbury Town, it was a rotten campaign, winning just eight of their 46 League One games and finishing the campaign bottom of the pile, 14 points adrift of safety and relegated to the fourth tier.
Castledine had spent the first half of the season as a regular under Paul Hurst and then Gareth Ainsworth before his time with the Shrews had to be cut short due to a stress fracture in his lower back.
But little did he know, after returning from his testing loan, that Boro had been watching.
Boro have a database of the Premier League's most promising young players, who they keep tabs on and monitor so they're ready to act if transfer opportunities ever present themselves. Castledine featured on that list.
The talent of a player who had captained Chelsea's Under-18s and Under-21s wasn't in question, though Boro were impressed by how he coped with his first taste of senior football.
But what also caught the eye was the manner in which he dealt with defeat and struggle. It's when the chips are down that any hint of a bad attitude can be exposed. Castledine never let his head drop.
His two League One loan spells strayed from one extreme to the other, but in Castledine's mind, his dazzling first half of the season with Huddersfield might have only been possible because of what he went through and learnt at Shrewsbury.
"Both were extremely beneficial," he said this week in his first interview with the club after signing a four-and-a-half year deal at Boro.
"The Shrewsbury loan taught me a lot, a struggling team fighting at the bottom of the table and then it gets completely flipped at Huddersfield. Everything I took [from Shrewsbury] really helped me. When times do get tough I'm able to dig in, I've been there before. That really helped my thrive."
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Castledine also learnt an awful lot from his dad, Stewart, who played in the Premier League for Wimbledon and with his son in the garden.
“I genuinely believe it made me a better player," said Castledine in an interview with the Athletic last month.
“It didn’t matter my age or size, I was never allowed to win, I had to earn it. I remember my 11th birthday party we were going to go paintballing but I could not do it because before the party he smashed me in a tackle and I sprained my knee. So when I got my first real win after several years against him, I was about 15, I knew it was real.
“We would do a lot of things. A goal would be set up and I would have to get round him to score, or there would be two little goals and play one-v-one for hours. Sometimes he would even angle his car to get the headlights shining on the lawn so we could carry on playing when it got dark. My mother would be calling us in for dinner and he would be the one replying ‘No, one more, one more!’
“I would not have had it any other way. It meant when I was up against boys my own age in the academy, I felt I had an advantage.
It should come as no surprise, considering that his dad played for the Crazy Gang, but Castledine doesn't just take pride in scoring or creating goals.
"I have always prided myself on the defensive side of the game, winning tackles and headers," he said.
"I like to think it's a big part of my game. It might not be conventional for a No.10 or attacker but it's helped me a lot. I'm not afraid to put my foot in and smash someone if I need to."
Expect to see Stewart in plenty of Boro away ends.
“I cannot remember the last game he missed,” Castledine told the Athletic.
“For example, I remember playing for England Under-17s at a European Championships qualifying tournament in Belarus. We played against them, Armenia and Slovakia. It was absolutely freezing. I would look across and he was one of the only people there standing on the side."
It's said there were only 19 other fans in attendance.
When it comes to choosing where he's going to live on Teesside, Castledine is more likely to lean on his mum for advice, however. Lucy Alexander presented Homes under the Hammer for 13 years. And she's not the only recognisable TV face in the family. Castledine's sister, Kitty, stars in Eastenders.
It wasn't just on the pitch where Castledine benefited from his spells out on loan. Prior to joining Shrewsbury, he'd only lived in London. He's only 20 but he should be able to quickly adapt to life on Teesside after his loan learning experiences.
On the pitch, Castledine scored 12 goals and created five more in 27 appearances for the Terriers.
Huddersfield tried to sign him on a permanent deal in the summer and again this month. They knew the latter was always a long shot, for Town were well aware of the strong Championship interest, as boss Lee Grant referenced in the statement the League One club put out when his Boro move was confirmed.
Several Championship promotion chasing sides were among those keen, but Castledine said: "As soon as I heard [he was wanted by Boro], I was interested straight away."
In conversations with Boro's recruitment team, it's understood that both Kim Hellberg and Adi Viveash agreed Castledine was a perfect fit for the side and style. Not only will he add a goal threat, but the coaches believe he'll offer something different to the other attacking players in the squad.
The timing was right and so was the fee. Even with a Chelsea sell-on included, £1m feels like a snip.
"Joining a club such as this can only benefit me," said Castledine.
"You can see the history of here of playing young players and giving them trust.
"That was a big factor for me. Looking at past players, there's always been trust in young players if they can perform."
Castledine was just 18 when he made his Chelsea first team debut, replacing Raheem Sterling late on in the Carabao Cup semi-final second leg victory over Boro two years ago this month.
Afterwards, John Terry posted on social media: "Love him and so will the Chelsea fans. He is like a magnet in the box."
Then Chelsea boss Mauricio Pochettino would regularly make the time to speak to Castledine, even when he wasn't training with the first team. When he did join in with first team training, he was wowed by Cole Palmer and once told of having to pinch himself after doing one vs one drills with Thiago Silva.
“Former academy guys that are now established in the first team like Reece James and Levi Colwill were great to me and the other youngsters," Castledine told the Athletic.
"They took us under their wing. They had been in the same position and knew what we were going through. They would tell us little bits like how to be when travelling to games, stressing to be early for training. They would sit next to me on the coach and talk to me, get me involved. They were really good.”
"It's difficult to be saying goodbye," wrote Castledine on social media this week in his Chelsea farewell.
But his thoughts are on the future, not the past.
He will, he says, play "without fear" at Boro.