ST. PETERSBURG, Fla. — If the Seattle Mariners are going to prove themselves true contenders, they need to stop giving away games to last-place opponents.

And that’s what this was Tuesday night: An 8-7 giveaway to the Tampa Bay Rays at Tropicana Field. They Mariners built an early four-run lead and, after nearly squandering that, built a late three-run lead.

It wasn’t enough.

A bullpen implosion by Nick Vincent and Mike Montgomery surrendered four runs in the seventh inning and snatched a victory away from rookie Edwin Diaz, who sparkled after replacing an injured Taijuan Walker.

But the collapse by Vincent and Montgomery was only part of the problem. The Mariners, despite scoring seven runs, wasted numerous opportunities to turn the game into a rout.

“We had a lot of opportunities tonight to score more runs than we actually did. We didn’t take advantage of everything. Their guy hung in there, and that proved to be the difference.

“We probably should have scored double digits tonight,” manager Scott Servais said. “It didn’t happen.”

The Mariners were 3-for-14 with runners in scoring position.

Even so, Vincent inherited a 7-4 lead to start the seventh. He gave up a one-out single and a two-out walk before yielding a crushing three-run homer to Evan Longoria.

“Good hitter,” Vincent said. “I looked at the pitch. It was off the plate a little bit. In that situation, it’s got to be a curveball. Now I know. So in the future, he probably won’t get that pitch again.”

Longoria’s homer tied the game.

In came Montgomery, who issued two walks on eight pitches before Corey Dickerson deliver a tie-breaking RBI single. The Rays protected the one-run margin over the final two innings.

“Mike, when he comes in,” Servais said, “it seems to take a pitch or two to get him into the strike zone. It took more tonight. The walks certainly hurt. Then Dickerson hit a ball hard into the hole.”

The Mariners are now a combined 3-10 against the American League’s three last-place teams: the Rays, Minnesota Twins and Los Angeles Angels.

The view from the other side:

“That was an awesome win,” Tampa Bay manager Kevin Cash said. “We haven’t had too many like that where we’ve come from behind. … We just kind of stayed the course.”

Walker departed because of what the Mariners characterized as posterior tibial tendinitis — inflammation in the tendon that attaches the calf muscle to the bones on the inside of the foot.

“It’s not as bad as I thought it was,” Walker said. “I’ve felt it before. It’s been feeling good the last couple of weeks. It just popped back up again. I hope to get it taken care of and be ready for my next start.”

Club officials viewed that as a better diagnosis than the original belief that it was an Achilles’ tendon injury.

“It was pretty obvious after that play that he wasn’t feeling quite right,” Servais said. “He had a hard time driving off his back foot down the hill. He tried to gut it out.

“He stayed in there as long as he could, but you could see that something wasn’t right. Right now, we are hopeful he is going to make his next start, but we’ll see how he is the next couple of days.”

Walker said the problem surfaced in the second inning and became worse after he handled a comebacker for the first out in the third inning. He exited after Dickerson’s one-out triple in the fourth.

Dickerson has been a nemesis for Walker. Dickerson hit a booming two-run homer in the second inning, after the Mariners built a 4-0 lead, and hit a grand slam against Walker on May 11 at Safeco Field.

The slam seemed to send Walker into an extended funk. He was winless in seven starts before dominating Cleveland in his last start.

Diaz replaced Walker and pitched 22⁄3 scoreless innings with five strikeouts.

The Mariners scored five runs in 52⁄3 innings against Tampa Bay starter Jake Odorizzi but wasted several chances for additional damage.

“We had a lot of opportunities tonight to score more runs than we actually did,” Servais reiterated. “We didn’t take advantage of everything. Their guy hung in there, and that proved to be the difference.”

The Mariners scored twice in the seventh against Dana Eveland and Ryan Garton (1-0). That made it 7-4. It wasn’t enough.