The first thing most observers notice when watching Jayson Tatum is the offense -- excellent footwork, tough shot-making and plenty of athleticism and versatility. That combination of skills makes for a great highlight reel.

It should come as little surprise to Boston Celtics fans that Brad Stevens wasn't watching highlight reels when he was evaluating the team's youngest acquisition, however.

"I'm looking at it more with a critical eye than the people that put together the highlights and see the one-foot fade aways and all that stuff," Stevens said in an appearance on Chris Mannix's podcast Monday.

Stevens added that while it's "great to be able to score in isolation," he evaluated Tatum on the other side of the ball.

"I was really impressed with -- and almost pleasantly surprised by -- his defensive ability," Stevens said. "I thought he did a good job of keeping guys in front. I thought he did a good job of using his length and guarding a number of different positions. ... Offensive stuff is going to come easy for him. Maybe not easy, but he has a natural scoring ability. (In Summer League) I wanted to see how he would pick up what we are trying to do defensively -- how his length would impact others, how he would pick up the off-the-ball schemes -- and I thought he did a pretty good job."

Tatum, of course, was the surprise selection by the Celtics during the 2017 draft. After Boston won the top overall selection, courtesy of the Brooklyn Nets, the assumption was that they would select point guard Markelle Fultz first overall -- a move which might have created some awkward questions for star guard Isaiah Thomas, who is due a big pay day next offseason.

Instead, the Celtics traded down to No. 3 where they took Tatum. He impressed in Summer League, but as Stevens noted, to earn minutes on a team crowded with talented wings, Tatum will need to prove that he can defend multiple positions in addition to scoring in a variety of ways.

That won't be easy for a 19-year-old, suddenly thrown into a league full of grown men considerably more talented than anyone he has faced before. While Stevens noted that the Celtics don't expect Tatum to cram six years of weight training in before the season starts in October, he will have to make some adjustments and learn by being thrown straight into the fire.

"There's going to be some times when you are going to have to guard someone stronger than you, and you're going to have to figure it out," Stevens said.

Interestingly, when discussing the help Tatum will have defensively in the Celtics front court, Stevens mentioned another rookie: Semi Ojeleye.

"We can mix and match a little bit," Stevens said. "If he's playing with Jae, Jae can guard up a spot. If he's playing with Semi, Semi can guard up a spot. If he's playing with Gordon, Gordon can guard up a spot."

Ojeleye has seemed like a lock to make the final cut, but while his money was guaranteed, his roster spot technically wasn't: The Celtics currently have 16 players with guaranteed money on the roster.

Stevens complimented Ojeleye's versatility during Summer League, however, calling it "enormous" and something the Celtics could use right out of the gate. Ojeleye is powerfully built and seemingly capable of playing any position between the three and the five.

In any case, Tatum seems like a lock to receive playing time in his rookie season at whichever position fits best.

"He's going to have to use his mind and use his length and compete," Stevens said. "That's just ultimately what you have to do."