I received an unexpected gift this past weekend.

An owner in one of my leagues, a Denver Post sports writer who knows his stuff, offered me Yankees closer Mariano Rivera for Pirates outfielder Chris Duffy.

I had to read the e-mail several times before I could believe it. Granted, Rivera was awful in the first month of the season, with an ERA over 10.00 and two blown saves in three chances.

But this is Rivera, the most reliable closer of the past decade-plus. And I could have him for Duffy, who isn’t bad and will steal 30 bases but is by no means a superstar.

In an e-mail a few days later, he explained himself, saying he needed stolen bases right away. Fair enough. Duffy is one of the 15-20 best base stealers in the game.

However, the owner also said this: “I’m sure I could have gotten more value a month or two later.”

Yes.

This is the problem with trading low. In the case of Rivera, it’s only one of the issues, because he’s bound to return to form and save 30-plus games. He’s simply worth more than Duffy, but it’s easy to get impatient and make quick moves you’ll later regret.

I know. I dropped shortstop Troy Tulowitzki when his average was under .200, deciding he wasn’t ready. Had I waited a week, I would have seen him start to put it all together. He’s a future All-Star, likely to hit 15-20 home runs this season.

So hold on to your April duds. In fact, collect them. A star hitting .196 with one homer (Carlos Delgado) through Wednesday will outperform Ian Kinsler (.302-9-23) over the final five months. Can you imagine Delgado not hitting 20 homers? Me neither. That means 19 are still unaccounted for:

Ready for liftoff

Alfonso Soriano, OF, Cubs

I’d trade almost anyone right now to get Soriano. Why? Because he has 40 home runs left to hit in five months. Or at least 38, since he hit his first two the first two days of May. He’s baseball’s only 40-40 guy, and that means he’s worth more than Jose Reyes and his 17 stolen bases (through Wednesday). After all, that’s 17 stolen bases out of his total he’s already achieved.

David Wright, 3B, Mets

Mets manager Willie Randolph said it best this week. He said he doesn’t sweat it when his players get in slumps, because “baseball is all about slumps.” “Today we’re talking about Wright,” he told reporters. “In a few weeks we’ll be talking about Reyes and Beltran.” Willie is a wise manager, and your fantasy team can benefit from this mentality. Ride out Wright’s slump, because he’s a young star with great potential who surely will be hitting .300 with 25 home runs at season’s end.

Gary Sheffield, OF, Tigers

Could Sheffy’s fine career be coming to an end? It’s always possible, but I still believe in him, because he’s still piling up the walks. The 38-year-old is still selective at the plate, and it appears his bat speed is still stellar. Remember, it’s been cold; he’s just getting warmed up. Here’s to a summer of Sheffield lacing extra-base hits all over Comerica Park.

Garrett Atkins, 3B, Rockies

This is the Rockies player I’d want on my team the rest of the year, over Matt Holliday. Not that Holliday isn’t a great hitter on his way to superstardom; my interest is in Atkins, who only heated up in June last year and has the swing to equal last season’s numbers (.329-29-120). He has a bunch of hot months ahead of him, since he has started the season slowly (.263-3-12).

Stalled

Ryan Shealy, 1B, Royals

He’s a slugger hitting .113-1-5 in 53 at-bats, but just as he was showing signs of emerging from his early-season funk, he hurt his hamstring. He’s eligible to come off the disabled list in the middle of May, but even if he does, there are just too many other 1Bs who are more productive to waste any more time on Shealy. Drop him unless you’re in a very deep league.

Coco Crisp, OF, Red Sox

He’s starting to swing the bat better, hitting over .400 the last two weeks through Wednesday. That’s great news, and there’s no question he’s in a great lineup. But he is one of a handful of players who are more popular in fantasy than they are valuable. At best in his career, Crisp was a .300-16-69 with 15 steals guy in Cleveland two years ago. That’s good, but outfield is a deep position in fantasy, and odds are you can do better. If Aaron Rowand is an alternative, he has greater power and more upside.

Notes: Astros OF Hunter Pence is the real deal, and Houston is counting on him to jump start a moribund offense. Rookies are always risky fantasy propositions, but Pence easily could be the NL Rookie of the Year. Grab him now if he’s still available. … The Pirates’ Ryan Doumit is swinging the bat really well and could steal playing time in the OF and at 1B. He’s the kind of super sub who is one injury away from a 15-20 homer season.