But the 21-year-old midfielder can now see light at the end of the tunnel after stepping up his rehab last week, and hopes to be back in action "sooner rather than later."
Longstaff was shortlisted for the Premier League's player of the month award for February and made 13 appearances in all competitions last term, scoring twice, before suffering the setback at West Ham in March.
He spoke to NUFC TV at United's Benton training ground on Friday after resuming light running and ball work, and admitted: "It's tough being out for so long - it's my first proper injury and on the back of probably the best two months of my life so far, to have it taken away was pretty horrible to be honest.
"But you've got to get your head around it as quickly as possible and start setting yourself little targets. Now I've hit one of them - I'm back running and kicking a ball - and it's given me a little bit more confidence to know it's alright and it's healing as it should be. It's a mental thing. It doesn't matter who you are, you're always going to have the negative thoughts at the back of your mind, so for me it's about trying to stay as positive as possible.
"It's a bit of mental battle with yourself and there's good days and bad days; sometimes everything gets a bit on top of you and then you have some really good days where everything feels great, so it's about staying as level-headed as you can, and on an even keel, and what will happen will happen. As long as you listen to the right people and take their advice, it should be alright in the end."
Longstaff limped off at the London Stadium after a 50-50 challenge with Robert Snodgrass, and explained: "At first I was in a brace for eight weeks, so for me it was learning to walk again and then learning to be more confident with my knee and put a bit more load through it, instead of having to compensate with all the other muscles around it - simple things you would do every day without thinking - then progressing from that to jogging and running and all the weights in the gym, and trying to get back as strong as possible. It's just about working hard and listening to the right people, and hopefully that will stand me in good stead.
"A knee injury is never something you want, especially at the time I got it - I was flying at the time and everything was going so well, and suddenly it gets ripped away like that. It's not nice but that's part of football, it's not like I'm the first player to ever get an injury. It happens every single day, all around the world. At the time it seemed like a blur - you go from the highest of highs and you've worked so hard for the opportunity, but it's football, it happens.
"It was an interview you did with Jonjo (Shelvey) the other day and I saw the video of the tackle; that was the first time I'd actually watched it back. I was trying to avoid it. It's something I don't want to see - it's brought me some pretty down times.
"It can be tough. With it being my first injury, everything's new to me. You get used to going into the physio room and it's new, and you're getting the treatment and it's new, and after a while it gets to you a little bit. Luckily my family were there for me. They keep me upbeat and happy and staying positive. Now I've hopefully done the hard bit and I'm on the exciting bit of working hard and getting back fit. The positives are hopefully going to outweigh the negatives in the next little bit, and I'm excited about that."
A return date has not yet been set for Longstaff, although the player is hopeful of being able to feature at some point in pre-season. He revealed: "It's something we've spoken about briefly but we haven't said 'I'm going to be back on this date'. We've got rough estimates but it just depends how the knee reacts to the stuff we do; when you start doing the running and changing direction, kicking a ball and sprinting, I think that's when you'll know if you're ready or if you need a little bit longer.
"There's no specific time on it; as much as it was a bad time to get injured, it's maybe been a good time as well because you get the whole summer to recover. Pre-season would be the ideal time but if it's longer then it is what it is. I was speaking to some of the lads and they just said 'make sure you get back when you're ready.' There's no rush - if it takes an extra two or three weeks then, in the grand scheme of things, that's not a long time.
"There's no point in coming back with doubt in your mind because you're never going to be able to play how you want to play and be yourself. So there's no specific time - it's just about seeing how it reacts and working hard every day, and hopefully I'll be back sooner rather than later."
