PHOENIX — It was a night for spotty intensity and some major irritation on the part of Boston coach Brad Stevens, whose timeouts were triggered by a little extra frustration.

But the Celtics are now also 3-0 on this western road trip, and overall on a four-game winning streak, thanks to Monday night’s 102-94 win over a Phoenix team lacking its only star, the injured Devin Booker.

Celtics rookie Jayson Tatum, right, slaps hands with center Aron Baynes during the second half of Boston's 102-94 victory at Phoenix on Monday night. Tatum led Boston with 23 points. (AP Photo/Ross D. Franklin)

But as the Celtics have discovered multiple times after taking big early leads this season, a letdown is almost inevitable. It even happened with Kyrie Irving, Marcus Smart and Daniel Theis (all sidelined by injuries now) in the lineup. The difference now is that Stevens’ threadbare mix of young players with several veterans sprinkled in have to fight against this tendency.

“Human nature hit us a little bit after our good start,” said Stevens. “We took our foot off the gas, which is hard not to do when you’re up 20 and the ball is going in early. But they keep coming. They did that last year and they’ve done that to us in the past. (Tyler) Ulis loves playing against the Celtics. At the end of the day you have to play the full 48 to win. Especially when (Abdel) Nader came in in the second half our energy level looked great, and that’s when we took off again.”

The power drain occurred over the last 9:50 of the second quarter, after the Celtics took a 20-point (39-19) lead on a Marcus Morris 3-pointer. The Suns, behind Ulis, out-scored the Celtics 26-9 heading halftime. Morris would leave for good at the end of the third quarter after aggravating a right ankle sprain.

“We relaxed a little bit. We thought the game was going to be easy,” said Al Horford. “One of the things we’re learning – and, we know, but sometimes you forget – is that teams are going to keep playing. They’re going to keep fighting. We just have to be better in that if we’re in this position again.”

Somewhat remarkably, though, the Celtics are suddenly gaining ground on the Eastern Conference-leading Toronto Raptors, who are now only three games ahead of the Celtics.

“For us it’s just to play good basketball at this point,” said Horford. “Make sure that with the group we have we’re playing the best we can, and get as much experience as we can and get as much experience as we can with this group, because we haven’t played a lot of games together with these lineups. So that’s all that matters for us at this point.”

Jayson Tatum led the way with 23 points, and Horford had 19 points, nine rebounds and seven assists.

Morris, who had missed Sunday’s win in Sacramento with a sprained right ankle, re-injured it after a foul and a crash landing following a 3-point attempt late in the third. He made all three free throws and didn’t return, finishing with 20 points off the bench.

But his worst fears were alleviated after some postgame tests.

“It’s just sore. X-rays came back negative, so everything is good,” said Morris. “It’s the same ankle I twisted (last Friday in Portland), so it’s a reaggravation. We’re going to see how it feels tomorrow when I wake up. I’m hoping it feels a little bit better and we’ll see if I can play (tomorrow).”

The injury didn’t get in the way of a personal milestone, though – Morris’ first three-game stretch scoring 20 or more points.

“I’m blessed then,” he said. “I can get past it. It came back negative so I could be out a couple days, but nothing long-term. So I’m good.”

In the meantime, the Celtics continued their search for roster depth, agreeing to terms on a 10-day contract with wing player Xavier Silas out of the G League after their intended target, Sean Kilpatrick, finalized a more lucrative deal with Chicago.

Boston closes its four-game road trip Wednesday night at Utah.

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