Dean Huijsen, Justin Kluivert and Antoine Semenyo struck the goals for Bournemouth on Sunday.
"We are very disappointed. I think we are playing so well, we create many chances but unfortunately we couldn't score," Lisandro told MUTV afterwards.
"We now have to adjust a few things, especially at set-pieces, we have to start games by scoring the first goal.
"We want to work on that, I think we have a great coach and a great group as well.
"We have the chance now because we know in this club, losing is unacceptable. We are here to change things."
Lisandro insists he can see a light at the end of the tunnel for the club.
"For sure we deserve more. I see a light in this club and I think we are going to achieve that," added Martinez.
"You see in the games that we are improving. We have to believe and for sure it is hard to accept this kind of result.
"I believe a lot in the staff and in our team that, how I said, we are going to change this."