He knows how difficult it is for young receivers to make that leap in their first seasons, even if they are drafted early like Matthews, whom the Eagles took in the second round.
"I think you see more zone at the college level than you do here," Kelly said. "And so, really being able to combat press-man [coverage], I think, is the biggest challenge for a young receiver coming into the league."
Since Randy Moss in 1998, only five rookie receivers have eclipsed 1,000 yards receiving in their first year - Anquan Boldin (2003), Michael Clayton (2004), Marques Colston (2006), A.J. Green (2011) and Keenan Allen (2013).
On average, 18 first-round receivers over the last five years have caught 43 passes for 592 yards and four touchdowns in their rookie seasons. Seventeen second-rounders averaged 27 catches for 408 yards and three touchdowns.
The numbers are significant but don't represent much of an impact. If Matthews finished with 27-408-3, though, he wouldn't be that far off from Jason Avant's 2013 statistics (38-447-2) out of the slot.
Kelly obviously believes that he can get more production out of the slot or he wouldn't have let the still-productive Avant walk away. Matthews will have a learning curve, but he's bigger (6-foot-3, 212 pounds) and faster.
And Kelly sees mismatch potential with Matthews quicker than most linebackers and larger than most safeties and slot cornerbacks. He also prefers a big inside receiver who can run-block.
Kelly wants big, rangy ball-catchers. During a recent interview with SiriusXM radio, Kelly noted how the Eagles don't have a receiver on the roster who weighs less than 190 pounds.
They actually have two in Damaris Johnson (175) and Jeff Maehl (184). But neither of those receivers is likely to make the team. Jackson, of course, weighed a generously listed 175 pounds.
Matthews has the size, but does he have speed necessary to break away from topflight defensive backs? He ran a 4.46-second 40-yard dash at the combine, but some NFL scouts say that straight-line speed didn't show up on his college tape.
"The big question was his speed and his ability to separate at the top of his routes," a senior NFC scout said. The personnel executive said that his team was split on whether Matthews could be a starting No. 2 receiver or strictly a No. 3 in the slot.
Not every slot receiver is a No. 3. Boldin and Colston have done most of their damage running inside routes. Matthews said he lined up in the slot for about 50 percent of his senior season at Vanderbilt. A good amount of his 112 receptions and 1,477 yards came on quick screens and yards after the catch.
There is more to being a great receiver - hands, body control, route running, for example - than speed.
"I don't think that really matters," Matthews said when asked about pre-draft concerns about his speed. "When I came to visit, Coach Kelly and his staff, they were complimenting my speed. They watched film. They knew how fast I was on the game field and with the ball in my hand."
The Eagles are being cautious. Veteran Brad Smith is still getting the majority of first-team snaps in the slot. But Matthews has gotten opportunities to go up against starting slot cornerback Brandon Boykin or safety Malcolm Jenkins.
Monday was the first time he was in full pads. On one play, Matthews lined up opposite the 5-foot-9 Boykin. It would have been nice to see if Matthews could have gotten separation against one of the fastest and best slot corners in the NFL, but quarterback Nick Foles went elsewhere.
A series later with the second team, Matthews caught three straight passes from Mark Sanchez - a screen and then two on short crossing patterns.
"I couldn't ask for a better situation," Matthews said. "Not only to be able to play in this offense, but to be able to go against . . . guys like Brandon Boykin every day."
Proof either way, though, won't come until Sundays.