Garner has not spoken to new United manager Erik ten Hag yet and is due to meet him the week beginning July 4 when the squad's internationals are scheduled to report for pre-season training.
Should Garner, who helped Forest to promotion, be omitted from Ten Hag's first-team squad he intends to secure a loan to a Premier League club.
"I'm not too sure (what will happen), personally. I've not had much time to think," Garner said after England Under-21s' 2-1 defeat to Slovenia last week. "I've gone from Championship football straight to international football; I've not really had a breather or time to think."
When asked if he would have a chance to impress Ten Hag in pre-season, Garner said: "Well that's the plan. That's what I'd like to do; go on pre-season and impress, it's all up to me, really. It's down to me as a player to impress him and hopefully make a mark.
"It's huge (next season). I feel like last season my career has only just started, I'm still young, just turned 21 couple of months ago. I've had such a good season this season, with the national team and at club level as well. As you said, it's very important now to hopefully play in the Premier League next season.
"I think I've come on leaps and bounds this season. Played a lot of games, 50-plus now, so I've developed a lot physically, which was the question I was asked - whether I'd be able to play, whether I'm ready physically. I think I've shown I'm physically ready to play and, like I said, felt like I've come on leaps and bounds.
"I just feel like, before I went on loan, there were questions of am I physically ready to compete, to play first-team football, and I believe that I've shown that this season. Not only playing a lot of games but how I've held my own in such a tough league and men's football, you know. I think I've shown that."