Allen Park — The Detroit Lions open training camp next week.

We've already explored the position battles we'll be watching, as well as potential breakout candidates. Today, we'll consider five players positions for a rebound in 2019.

Matthew Stafford, QB

Trying to assess how much blame Stafford deserves for last season opens us up to larger debates about the quarterback, but circumstances were certainly less than ideal. His top tight end was cut before the season and not adequately replaced, then his most-productive receiver was traded at the deadline. Finally, injuries, namely to Marvin Jones and Kerryon Johnson, caused offensive production to flat line down the stretch.

On top of all that, there was an offseason report the back injury that nagged Stafford during the second half was actually a broken bone.

So, yeah, there are some built in excuses for Stafford's worst statistical year since 2014.

Of all the numbers, most troubling was his efficiency. His yards per attempt and yards per completion were his worst since 2010 and 2012, respectively. And the per attempt figure ranked 27th among qualifying quarterbacks.

This year, with an upgraded set of weapons, Stafford is poised for a rebound. The upgrades at tight end, and the return of Jones and Johnson to the lineup, will be key. Even with Detroit's added emphasis on running the ball, Stafford should return to the ranks of passers throwing for more than 4,000 yards, but even if he doesn't, he should be able to make more of each throw.

Jamal Agnew, CB

Agnew was an All-Pro punt returner as a rookie, and had worked his way into a bigger defensive role his second season before a knee injury shelved him 10 of the final 11 contests.

In the five games he played, his punt return average plummeted from a league-best 15.4 yards in 2017 to 4.8 yards. To be fair, he had little to no breathing room provided by his blocking, and when there was daylight, it was often accompanied by a yellow flag negating the positive, including a touchdown return against San Francisco early in the year.

Agnew is far too good as a return man to repeat the dismal performance, and a new special teams coordinator should help clean up the blocking and penalties.

Danny Amendola, WR

At first glance, Amendola's 2018 production wasn't far off from his career averages. During his brief, one-year stint with the Dolphins, he caught 59 balls for 575 yards. But again, when we look at the efficiency numbers, we see a decline.

His yards per target and yards per reception were both four-year lows. He finished the year with one touchdown and fewer than half of his catches resulted in first downs, an uncommon feat for the receiver position.

It's fair to ask whether age is catching up with the veteran pass catcher. He will turn 34 in November, after all. But he's one of many players this time of year who insist they're currently in the best shape of their lives. We'll give him the benefit of the doubt on that front, and given the way Golden Tate transformed into one of the league's most productive slot receivers while working with Stafford, it bodes well for Amendola to have a late-career resurgence in Detroit.

Sam Martin, P

For the first four seasons of his career, Martin got better every year, peaking in 2016, when he delivered the best season the franchise had ever seen at his position. The following year, an offseason injury sapped the power in his leg, and he continued to fall short of the lofty expectations he set for himself last year, when he finished 23rd in net average.

Martin's best years came when John Bonamego coached the special teams and the experienced assistant is back at the helm this year. If anyone can get Martin back to the top of his game, it will be Bonamego.

Theo Riddick, RB

For years, Riddick has been one of the league's best receivers out of the backfield. In 2015, he set the franchise record with 80 receptions at the position, and has continued to be one of the team's go-to weapons in the years since, especially on third downs.

Riddick caught 61 passes last year, the second-most in his career. But it's what he did after the catch that's concerning. Long one of the league's most elusive players, he wasn't fooling defenders the same way in 2018.

After averaging nearly 24 missed tackles as the receiver the previous three seasons, he only dodged seven a year ago, according to Pro Football Focus. That, along with a career-high five drops, contributed to a five-year low in first-down catches (16).

At just 28 years old, it's unlikely Riddick has lost a step. The greater possibility is opponents have keyed in on some tendencies, both personal and with the scheme, which require adjustment. His track record, both as a receiver and pass-protector, should afford him the opportunity to reestablish himself as a premier third-down back.

jdrogers@detroitnews.com

Twitter: @Justin_Rogers