And Lassad told Celtic's website: "I'm absolutely delighted to sign for Celtic. I hope to play my part in bringing success to the club."
After beginning his career in France, Lassad joined Deportivo from Chateauroux in 2008, initially playing for the Spanish club's B-team.
He graduated to the first team the following year, in which he also won the first of his three international caps.
Miku and Ambrose joined Celtic on Friday
"Celtic is a huge club with a magnificent history and a fantastic support," added Lassad. "I know all about the wonderful atmosphere at Celtic Park and I can't wait to play there.
"I know that Celtic already has a great squad of players and I'm very much looking forward to joining up with my team-mates.
"And, of course, I'm also looking to competing in the Uefa Champions League with such a great football club."
Lennon completed the permanent signing of defender Efe Ambrose from Ashdod as well as bringing in Getafe forward Miku and Nantes goalkeeper Lubos Kamenar on loan before the closure of the summer transfer window.
"Efe is a very powerful centre-half, a bit in the [Victor] Wanyama mould," added Lennon. "A little bit taller and maybe a little bit more dynamic but he can play either in centre midfield or centre-half.
"We've wanted him for quite a while so we're delighted to get that one over the line.
"Miku's a player that we've known about for a while but we didn't envisage any chance of getting him. We got information three or four days ago there was a possibility we could do.
"Again, a big signing for us and he's got proven pedigree in La Liga and he'll compliment [Gary] Hooper very, very well.
"We needed to get a couple of bodies in but we needed the quality.
"[They're] probably not the marquee signings everyone was looking for but we made it clear two or three weeks ago we wouldn't be doing that because I like the atmosphere in the squad and I like the way they compliment each other.
"These guys are hungry and you're always looking for hungry players."
Celtic could have been three or four goals ahead at half-time against Hibernian on Saturday.
Celtic dropped two points against Hibs
Striker Gary Hooper twice hit the woodwork and Tony Watt missed a one-on-one with keeper Ben Williams.
After the interval, Hibs were much improved but Celtic had chances through Paddy McCourt and Filip Twardzik to add to Mikael Lustig's strike and James McPake's own-goal, but neither could find the target.
Lennon told BBC Radio Scotland: "I am not disappointed with the performance at all; I'm annoyed at the two goals that we conceded.
"The performance was decent. Some of our attacking play in the first half was sensational.
"We should have won the game."
The Celtic manager was angered by the mix-up between his goalkeeper Fraser Forster and defender Lustig for Hibs' first, which was scored by Tim Clancy.
"The players are left in no uncertain terms what is expected of them," added Lennon.
"It was a poor goal and the second one was a poor goal.
"If you concede goals as poorly as we did sometimes you don't win the games."
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