Lallana has not started a game since March having injured his hamstring against Crystal Palace and though he made a couple of substitute appearances before the end of the campaign, he was not chosen to go to the World Cup with England.
The 30-year-old's last competitive appearance for England came in June 2017, before he suffered a thigh problem that kept him out of the first part of the 2017-18 season, and Henderson thinks Liverpool and England would benefit from having Lallana in their starting line-ups.
Lallana has 34 caps for England and was named the nation's player of the year for 2016, yet he had to withdraw from the international games in September due to a groin injury and has not played since.
Once he returns to full fitness, Henderson feels he could provide Gareth Southgate's side with a creative spark.
"He is such a big talent - not so long ago he was England's player of the year," Henderson told reporters.
"He is a massive player. I feel that we have missed him, both with Liverpool and with England.
Minor setback this week, which isn’t ideal as I’ve been in great shape until now, but it is positive news it’s only a brief layoff. Pleased I’ll be back and available for Liverpool, both domestically and in the Champions League, during September and then England again for the next break. I know I can make a significant contribution over the course of a long season and that’s where my focus will remain as there’s so much to play for. Doing my best and giving everything for my teammates, management and supporters of club and country is all that matters. Back at Melwood now making sure the time off the pitch is as short as possible, so will be supporting Gareth and the boys from up here. Good luck lads and keep making the country proud.
"It's exciting for me to know that when he comes back in, he can take us forward even more. He can help us get better and I am excited for Liverpool and England for him to get back playing football and to show everyone what he is capable of.
"There's not many players that play like Adam. He is unique in that way, how he links the play. He's so good on the ball, he's got a great work ethic and he still does the defensive work.
"We have missed him in the sense of what he brings to the team. It would have been a boost to have had him at the World Cup. And for Liverpool too.
"Last season would have been even better for both teams because of the talent that he is.
"But if he stays fit he can be a massive player for us going forward and still have a big impact."
Henderson and England face Spain on Monday in Seville in the Three Lions' third match of the Nations League.