Smith will play his first NRL game since rupturing the anterior cruciate ligament in his left knee against the New Zealand Warriors last September when he runs onto ANZ Stadium to face St George Illawarra on Sunday.
Tahu is in Cape Town with the NSW Waratahs.
And Parramatta have won just three from seven.
"I don't think anyone's realised until this year how big a loss he was, not only as a player," Smith said of Tahu.
"By the end of last year he was probably the best centre in the world.
"What he does off the field as well, just in terms of his professional attitude towards what he does - recovery and preparing for the game and training and everything.
"They're probably things that the fans and a lot of people don't see.
"It's a big loss."
Parramatta's centres have been a work in progress this year.
"It's just been a bit of a disappointing start with Inu injuring himself and with the team not going too well and it's easy for everyone to start looking at why and maybe what would happen if Timana was still here," Smith said.
"We've still got a fair while to go and we can only improve."
Inu and Hayne are expected to be the Eels' long-term pairing with 108kg Smith, who signed with the Eels for a further three seasons earlier this season, slated for a move to the back-row.
His more immediate concern, though, will be stopping Dragons opposite number Matt Cooper.
"I've come back at the wrong (time). I would have liked a game or two to get into the flow of things before I come up against these sort of guys," he said.
"I'd say you'll find him in the (NSW) Origin team, he's a class player and he's up there with the best (centres)."
Smith, who also underwent off-season surgery for a shoulder problem, has played one game this year for Eels feeder club Wentworthville.
"NRL's not the type of comp where you can go out there and cruise through it," he said.
"I guess I've got to go out there and I've got to rip in but I'll just be happy to get through it and get the year going for me. It's the start of my year."
The Eels will aim to build on last week's win over Penrith while the Dragons come off the bye and their Anzac day upset of the Sydney Roosters.
"In a total of 80 minutes there were 15-20 minutes where we played good football and we sort of got away with a win," Smith said of the 26-18 win over the Panthers.
"We need to put together more constructive sets that have purpose if we're going to be up there by the end of the year so we're able to rumble with the big boys."