Neymar, 28, has been linked with a return to Barca effectively since he joined Paris Saint-Germain for €222 million in August 2017, with speculation again building around a possible reunion this summer.
Six-time Ballon d'Or-winner Messi, 32, told Mundo Deportivo (h/t Sport) he and his team-mates did their best to convince the star to stay but that Neymar is itching for a comeback:
"It bothered me at the time and we tried to convince him not to do it. But we all want to win and have the best players around us. He's one of the best players in the world and he brought a lot to the team. But it's understandable that people would see it as leaving in a bad way."
"He is desperate to return and has always appeared to regret his decision. He did a lot to come back and that would be the first step in making it happen. He was always fine at Barca, performing at a very high level. He's a different type of player who is difficult to stop one-on-one and scores goals.
Neymar has scored 67 goals and recorded 39 assists in 77 total appearances for PSG, winning two Ligue 1 titles and with a third seemingly on its way as they lead this season's table by 10 points.
Their domestic dominance hasn't transitioned into European success, however, and the French side have fallen in the UEFA Champions League round of 16 in each of Neymar's first two campaigns at the club.
Optus Sport illustrated how Barcelona haven't been entirely smart with how they've invested their funds from the Neymar sale, including the £105 million purchase of Philippe Coutinho (now on loan at Bayern Munich):
Barca would likely be required to spend an almost-identical—if not greater—sum than the one they received to re-sign Neymar, although his contract is due to expire in 2022.
Neymar returned from a four-match injury lay-off when PSG lost 2-1 at Borussia Dortmund in the Champions League last 16 on Tuesday.
His comments to RMC Sport (h/t Sky Sports) after the game suggested there's tension at the Parc des Princes:
"It is difficult to spend four matches without playing, but, unfortunately, it was not my choice.
"It was a choice made by the club, the doctors. They took this decision and I did not like it. We had lots of discussions about this because I wanted to play.
"I felt good but the club was scared. In the end, I am the one who suffers."
It might be wiser of Barcelona to sign Martinez, not least because Messi appears to be a major fan of the Inter frontman, who has also scored nine goals in 17 games for Argentina.
Messi lauded his compatriot's quality and added:
"He's spectacular. He's got a lot of impressive qualities. I can see he's going to become a great player, he's exploded on to the scene now and is showing what he's capable of. He's very strong, beats his man and scores goals. He'll fight with anyone in the box, holds on to the ball, and can produce moments by himself. He's got a lot of quality and is a very complete player."
When asked whether Martinez could form a fruitful partnership with him and Luis Suarez, Messi replied:
"It would be incredible to all play together. I hope that they could all come here and we could fight for everything, including the Champions League. Lautaro has a lot in common with Luis, both are ready strong and use their bodies well, holding on to the ball. They both score goals too. They are very similar."
The 22-year-old has enjoyed a breakout campaign at the San Siro, netting 16 times in 22 outings across all competitions, two short of matching his career-best total (18 goals for Racing Club in 2017-18).
The club announced on Thursday that they had completed the emergency signing of striker Martin Braithwaite from Leganes on a four-and-a-half-year deal. His contract comes with a €300 million (£251.7 million) release clause, with the deal made possible after Ousmane Dembele was ruled out for around six months. However, Barca will likely look for a more elite option to replenish their attacking corps for the long term.
Martinez's international record includes an impressive run of eight goals in 13 games for La Albiceleste across 2019, and Statman Dave highlighted his attacking prowess in Serie A:
Juan Jimenez of AS wrote in December that Barcelona hope to sign the South American, whose release clause will be €111 million (£93 million) during the first two weeks of July 2020.
Suarez has a contract at the Camp Nou until 2021 and will need replacing in the near future, with Messi seemingly clear over whom he would like to see take over his position in attack.