And Johnstone admits himself he would never have predicted the impact Rovers would have on him.
The young Manchester United keeper arrived faced with the monumental task of filling the gloves of Ross Turnbull, a contender for Rovers’ star man up to that point.
And fill those gloves Johnstone did, even managing to keep Turnbull out of the side when he eventually returned from injury.
Fresh from signing a five year contract at Old Trafford, Rovers knew they had landed a potential star of the future.
And over the following three months, it became clear his chances of fulfilling that potential were pretty good indeed.
Lauded for his performances, Johnstone told the Free Press he loved his time with Rovers, a club that he says he now holds great affection for.
“It was a fantastic few months for me,” he said.
“Obviously there was the big disappointment of relegation at the end of it but on a personal note it was great.
“I played well throughout and got to play plenty more games than I expected.
“I didn’t expect to be coming to Doncaster and playing so many games.
“I knew I was coming in for the initial month to cover Ross, who I knew had done really well all season.
“When the end of that first month was approaching, we knew Ross was still struggling so it was extended.
“But even when he got back fit, I stayed in the side and he was on the bench a few times.
“It was massive for me to go and play in a side with plenty of quality.
Johnstone was full of praise for Turnbull as well as goalkeeping coach Paul Gerrard for their help during his time at the Keepmoat.
He said: “Those guys really helped me during my time in Doncaster with plenty of advice and support.
“Ross was really supportive all the time and encouraged me plenty.
“It helped a lot.
“I got on really well with the lads and they helped me settle in quickly.
“I spent quite a lot of time with David Cotterill because he lives near Doncaster so we’d hang out when I was staying over.
“The other loan lads like Billy Sharp and Enda Stevens were great as well.
“I really enjoyed it.”
Johnstone reported back to Manchester United regularly during his time with Rovers and coaches were pleased with his spell.
He said: “I was speaking to Chris Woods, the goalkeeping coach, all the time.
“They were really pleased with what I was doing and that I was playing so many games.
“They kept a close eye on me all the time I was there and had someone at the majority of the games watching.
“It’s good to know they’re not just a club who throws you out on loan and leaves you to it.
“It was a really valuable move from both sides of it.
Johnstone says he was devastated with Rovers’ relegation, bringing his loan spell to a disappointing end.
He said: “It was absolutely devastating for everyone.
“I was gutted that it ended the way it did
“That last day was horrible really.
“We heard the Doncaster fans cheer in the second half so we knew it was going our way at Bolton but it wasn’t until the end that we knew it’d gone wrong for us.
“It was never a penalty for Leicester but you can’t change things.
“We were so close to staying up but it just slipped away.
“We picked up some really good results in March against Leeds, Sheffield Wednesday and Huddersfield but the run after that was poor.
“Losing to Birmingham and Bolton in games when we could easily have got something was a big blow and we never really recovered from that.
“We took it down to the last game but we left too much to do.”
Johnstone’s Rovers spell also concluded on a more positive note with a call up to the England U21 squad.
But he suffered a blow last weekend when he forced to pull out with a knee injury.
The keeper was due to be part of Gareth Southgate’s plans for the European Championship qualifier with Wales plus the upcoming Toulon Tournament but was forced to withdraw.
He said: “It’s gutting for me because it was a big opportunity.
“It’s an injury I’ve been carrying for a while so I knew it was there.
“I was playing with it during my time with Doncaster but it’s been decided that I should rest it ready for next season.”
A Rovers return for the keeper next season seems unlikely following relegation to League One.
After his consistently impressive run in the Championship, few would begrudge the 21-year-old from setting the second tier as the minimum level for next season.
“I’d like to be playing in at least the Championship next season,” Johnstone reluctantly said.
“I’ve done well at that level this season so it would be best for me to at least stay at that level.
“If Rovers had stayed up then I would definitely have been interested in coming back, if United were happy with that.
“Going to Doncaster and playing so many games was brilliant for my development and I want to kick on from that next season.
“I don’t really know what is happening with my future at the minute because of the change of manager at United.
“There’s going to be a lot of changes over the summer and I’m sure the new manager will want to assess everything before any decisions are made.
“I’m excited about what is to come after these last few months with Doncaster.”
Few could have predicted the impact Sam Johnstone would have on Rovers when he arrived at the club on loan on the final day of January.