The Southampton team captain suffered a knee injury in the win over Reading at the start of December, so was ruled out of his side's Christmas fixtures.
However, having been in light training for the past few weeks, Lallana joined his teammates for their session at FC Barcelona's Ciutat Esportiva Joan Gamper on Wednesday afternoon.
After Saints' first day in Barcelona, Adam spoke to Saints Player
about his injury and his desire to make a good impression on new First Team Manager Mauricio Pochettino.
"Getting injured was obviously frustrating. I feared the worst when I it first happened, so I was lucky that I've only been out for weeks rather than months," he said.
"I've just been professional about it – got stronger and worked harder so I hopefully come back a fitter and better player.
"With this being my first session back, it was nice to be with the lads again and trying to impress the new manager.
"I felt a little bit rusty but it's my first day back after being out for six-or-seven weeks.
"It'll take me a few training sessions but hopefully I haven't lost too much fitness because I've been doing quite a lot of running."
Overall, then, it was an enjoyable first full day in Spain for Lallana, with the squad visiting Barça's Camp Nou stadium in the morning.
And the 24-year-old insisted that he hopes yesterday's trip to the largest stadium in Europe won't be his last – and that his next visit won't be as a tourist.
"One day I want to be playing there," said Lallana. "You've got to set your sights high because you never know where the club could be in five years.
"We've come so far in a short space of time, and if we continue at the speed we've been going then you never know.
"It's been a nice first day. Visiting the Nou Camp was quite surreal, and the facilities at the training ground were great.
"The Chairman, Nicola Cortese, has played his part in it. When we go away, the trips are always top-drawer – five-star this and five-star that.
"We train at the best facilities and eat the best food, so we're well looked-after and that's only going to help us get better."
Lallana now turns his attention to breaking back into the team with them on such a good run of form, having lost just one league game in their skipper's absence.
He said: "The lads have done absolutely brilliantly over the Christmas period, and the game on Monday [against Everton] was outstanding – the first half was probably the best I've seen all season from the stands.
"I'm so eager to get back playing, especially under the new boss. There's a team of four who have come in, and the gaffer is starting to get his messages across.
"We've come here for four days so he'll start to tactically put his imprint on how he wants us to play.
"You saw what style of football he wants us to play on Monday night – the lads came flying out of the traps and were pressing hard.
"It's going to be different, but it's going to be good."