Selected in the third round of the 2013 draft, the Cal product was at the bottom of the depth chart after uneven play during training camp, but his number was called in a Week 2 contest after veteran receiver Malcom Floyd suffered a scary neck injury.
Allen responded with an 18-yard reception two plays later, finishing with two catches for 34 yards.
It wasn't exactly a breakout performance, but Allen showed that he belonged and used that effort as a springboard toward an impressive rookie season, finishing with 71 receptions for 1,046 receiving yards and eight touchdowns.
"That's really where it started, back in '13," Chargers offensive coordinator Ken Whisenhunt said of Allen. "Malcom got hurt, he went in and played and he made a catch in the game on a route he had been awful at up until that point.
"That kind of turned the tide. You saw him grow up a lot. He really worked hard. One of the things I really love and appreciate is how hard he works. He loves the game. You see him on the practice field working that way and that's what you want to see -- one of your best players practicing at that level."
Allen is ninth in the league in receptions with 19 for 196 receiving yards and a touchdown through three games, but he has just 70 yards after the catch, No. 41 in the NFL.
As a team, the Chargers are No. 14 in the NFL with 318 yards after the catch. The Chargers were No. 7 last season with 2,081 yard after the catch.
The Chargers have just one passing play of more than 40 yards this season.
"I'm most definitely trying to attack the YAC (yards after catch)," Allen said. "It's being a baller. Once you get the ball you have to turn into a punt returner or runner, and just go at it."
The Chargers are playing against an Eagles defense giving up 266 passing yards a contest, No. 25 in the NFL. Philadelphia's defense has given up 460 yards after the catch, third-worst in the NFL.
One of the ways the Chargers will attack the Eagles is by moving Allen around. The Cal product has seven receptions from the slot, 12 receptions from lining up on the perimeter and has even lined up as a tight end this season.
"With a diverse group that we have, it's hard for teams to just key in on him -- with his speed and with the other guys, with the tight ends," quarterback Philip Rivers said. "If we can continue to improve running the ball, it makes it harder for a team to just take him out of the game because he's in so many different places. He doesn't line up in the same place on two consecutive plays very often."